The Arboreal Tank

Tree frogs spend most of their time up high in tree branches in their natural habitats, and therefore are much better off in taller tanks which better suit their instincts. Personally, the best set-ups I have seen have been using the tall square or hexagon shaped tanks with branches for climbing.

Tank Size
The size of tank you will need largely depends on the type of frog you will be housing in it. Smaller anurans do well in smaller sized tanks, but as the number of specimens increases, so does the tank size requirement.
For example, small treefrogs can do well in as little as 20 x 20 x 30 inch (50x50x75 cm) tank.
As a general rule however, most species do really well in a 20 gallon tank.
If you can't afford an arboreal tank, you might try a "combination tank". Combination tanks are normally 25 gallon aquariums, which measures 18 inches long, 24 inches high, and 13 inches deep. ***Scout your local paper for good deals on used tanks and lids.

Humidity
Many people make the mistake of thinking that if a frog feels dry they'll hop over to the water supply and take a dip. Much as I love the critters, you can't give them the benefit of the doubt when it comes to this type of intelligence. Some species are very dependant on a humid environment. This can be achieved in several ways. First of all, get a spray bottle!! Keep stale water in it and spray down the tank regularly. Spray everything! Live plants are especially good and retaining the water for some time, though if you have plastic ones it certainly doesn't hurt to spray those too.
(Note: this is not always necessary for certain types of toads which prefer dryer climates)
The other thing to do is to put down moss down moss and keep it wet, wet the soil, or if you don't have soil, you can obtain a moisture retaining product called vermiculite at most garden stores which works really well too.
A secure tank top that keeps the tank from drying up, while still allowing for ample ventilation is a really good idea. Some tank set-ups get a nice green-house like quality which is ideal for certain species of frogs.
If you have a set-up like I do, which has running water in it, you usually don't have to spray the tank down as much because the running water splashes a bit and creates the same effect. This is, of course, dependant on what species we are talking about.
If your frogs require a very humid climate, you may want to try getting an airstone to put in the water. The rising oxygen bubbles will be pretty good in driving up the humidity levels. Ask your pet dealer for more info.

Tank Top
The main issue with finding an appropriate covering for your tank is 2-fold. On the one hand, preventing the escape of a beloved frog pal is the ideal, and on the other, good ventilation is necessary for the health of your pet.
First of all, soft screening is really good! I strongly recommend it for 2 reasons. One, the soft kind (i.e. not the kind that doesn't bend a little bit when you push on it) will prevent a lot of overactive frogs from getting injuries when hopping up against the ceiling. The other reason, of course is that the screen provides great ventilation. There are a number of screen cage lids that are available in your local pet stores that are perfect for frogs. Make sure the lid is on very secure...not to make the frogs sound like Superman, but you'd be amazed at the amount of letters I get about frogs hopping up so often that they actually opened it and got out!
For frogs that require a sort of greenhouse humidity, be sure to leave some opening in the cover that is screened for ventilation.

Lighting
This is still a subject of a lot of debate. Generally speaking, most experts say that unlike lizards, turtles, and snakes, and other herps, frogs don't need what's called "full spectrum lighting" to be healthy. Full spectrum lighting aids some animals in producing Vitamin D3. Frogs get this vitamin through their diet, so only ordinary light is required. Not that this type of light, which can be provided by using special types of light bulbs available in most pet stores, will do any harm as long as it's limited to a maximum of about 4 to 5 hours a day.
Generally speaking, though, the best rules of thumb are:

Don't use a light that's too bright or you could damage the frogs' vision. Even if it's not bright enough to damage their vision, a light that is too bright for their comfort will mean frogs that hide all the time, which fairly boring for the keeper.
The most suitable type of artificial lighting for most frogs is fluorescent. For one thing, they tend not to produce too much heat. A lot of frogs will end up jumping on to the light bulb, which can be very damaging if the bulb is really hot. If the type of lighting you choose tends to get very warm, make sure to find a way to keep the frogs from landing on the bulb.
Some frogs use daylight to know what time of year it is etc...just as they would in nature. A good daylight period is around 12 hours. Of course, it's even better if the tank gets some sort of daylight in the room.
You probably don't want to have direct sunlight on the tank though, especially since it means algae is going to go rampant on your aquarium walls, and make cleaning the tank a real pain. (not to mention the fact that it will need the cleaning much more often)

Heating
If you have a frog that requires a temperature different from the climate you live in, you will need to look into options for heating your tank. If you have a fairly substantial water area in your tank, you may need to control the water temperature as well.
Heating the Air:
The best option is simply to warm the room to the appropriate temperatures. In this case, you may want to leave the water slightly cooler, so that they have a place to escape the heat. If this isn't an option, you have a few other possibilities.
One is to get a Heating Lamp. These seem to work ok, as long as the lamp isn't inside the tank. If you place the bulb in the tank, you will need to customize it to make sure your critters don't fry themselves on it when they jump on it (as they seem to inevitably do in these cases.) You'll probably want to keep a regular timing schedule with the heat lamp to maintain the usual fluctuations that would normally occur.
Another option is to get a Heating Pad. This basically is a pad that you put under or around some part of the outside of the tank which warms it. You don't want it to cover the entire underside of the tank as the frogs need to be able to escape the heat if they want to. NEVER put the heating pad, or any other heating device inside the tank. Amphibians will burn themselves very easily on them.
Heating the Water:
There are a variety of heating devices for fishtanks available in pet stores. The most common is the simple thermostat-controlled glass tube heater that fits onto the side of the tank. You can set a temperature for them and everything is nice and automatic. They even have tiny ones that fit into my mini-plastic tank for the Dwarf frogs! The main problem here is that these tend to require a high water level to function properly, which doesn't leave you with a lot of room for the land setup. The other option is the fully submersible heater, which although more pricey is worth the dependable temperature control it offers, and is nice and waterproofed with well coated cords so you don't run the risk of electrocuting your froggies. Just be sure that there aren't any huge gaping holes in the top of the tank through which your pet can escape.
Oh, and one other thing...most heating systems heat the tank to "4 degrees higher than the temperature it started out on" ... and then you have to adjust it over the next couple of hours (which may be many hours, if your dealing with a larger tank size) so it's very, very recommended that the frogs are not put into the tank until you have the heater figured out (generally speaking, put the heater in 2 days before you get the frogs) because there sometime is a good deal of fiddling with the temperature that goes on before it gets to be "just right."
You will need to seek advice from your local pet dealer for more info on heating issues.

Substrates
The easiest option is to use gravel. It's easy to clean and comes in lots of different colors and sizes, and it's re-usable. Make sure to clean the gravel really well before lining your tank!
Frogs have been known to accidentally swallow the occasional gravel stone, but this is harmless and generally the stones just pass right on through. You gotta figure if everything frogs eat crawls on the ground then they're bound to get some ground every now and again anyways! However, if you're worried, I hear Pine bark is a good choice (surprisingly, I've heard that the best kind is actually NOT the kind you find at the pet store but the generic kind you find in plant stores The kind in the Pet Stores tend to be dusty and the chips are really small.) Cedar or pine shavings, by the way, will have the same potential problems as gravel or sand....so avoid these if swallowing the little stuff is a concern.
You can also find nice stones to line your tank. The main requirement is that the stones aren't sharp, leaving areas where frogs can scrape against them and wound themselves. I use glass marbles to line the tank of my african dwarf frogs! I wouldn't recommend this for any land portion of your tank, however.
Sand also works well and makes a nice soft setting. This is especially good if the animal requires a slightly dryer environment, like many toads. It's cheap, and you can always compliment it with a few rocks.
Common potting soil is a pretty good option as well. It's disposable, easy to find, easy to keep moist, and isn't expensive. The coolest thing about it is that you can plant real live plants in it and it will be really close to natural surroundings for your pet. You gotta be careful setting up the water portion if you're using the soil option so you don't end up with a lot of mud though.

Decor
Moss makes an excellent compliment for the land portions of your terrarium. My firebellies have a gravel substrate in their tank, but I cover the gravel with dried moss that can be bought in most pet-stores and which was wetted in clean stale water. I just have to make sure that the moss stays a little moist and check that there isn't any mold growing on it, but the frogs absolutely LOVE it!
Plastic plants and decorations can be obtained from most pet stores. Particularly cool are plants with suction-cups for sticking to the glass. Re-arranging the decorations when you clean seems to make the frogs happy...(I guess even frogs get bored)
Hollow rocks or logs that give your frog pal a place to hide are very recommended. I have a giant hollow squid in my Firebellied Toads' tank and they hide there all the time!
If you're really energetic, you can plant some live plants and make your terrarium totally awesome! Go to your local plant store and start looking for nice plants that will do well in your tank! It's a bit more work, but it can have really impressive results. (I have never read in any of the guides which plants are good and which are not, but I imaging the list is quite huge. Your best bet is to use common sense, i.e. don't try planting prickly cactus, and it will probably be just fine.)
For those tree frogs, don't forget a thick tree branch for climbing. (As for those fancy branches at the pet store, cut one out of your own yard. Scrub the branch with antibacterial soap, then freeze it for forty-eight hours. Freezing the branch is imperative. This will ensure that all harmful bacteria and parasites are killed.)
A colorful background is also enjoyed by frogs. I have a tropical background with lots of greens, blues, browns, and some red in it. You can make your own or buy one. Contact paper and construction paper both work well. Just make sure the background has an even mix of greens, blues, and browns. If you have too much of one color, your frog can actually be affected in a negative way.

Filtration and Cleaning
Cleaning the tank:
There are several things you need to keep in mind here. The main thing is that despite the fact that this can be an annoyance, it's not something to procrastinate on. A dirty environment is almost always the cause of illness in frogs, and once your frog gets sick it can be very difficult to save it.
Regular tank maintenance includes a scrub-down of all the items in the tank (excluding the frogs) with warm water. Yes, that means the plastic plants too...frogs leave residue on everything, including the glass walls, which for some species can be toxic when it builds up. Make sure the glass gets cleaned regularly as well. The other thing to look out for is mold, particularly in moss etc, and dead bugs. Moss is really really cool for frogs because it's soft, retains moisture, and is pleasant to hang out in, (well, if you're a frog anyways) but it needs to be cleaned regularly! You can clean it with water (preferably stale water) Though it may be worth the extra buck fifty or whatever the package costs to simply replace the stuff every now and then. Keeping the water clean:
Cleaning an aquarium tanks is basically the same for frogs as it is for most freshwater fish, except it tends to be required more stringently because frogs shed more often. The biggest problem with the half and half set-up is figuring out how to get a filter that works without a full tank. There are undergravel filters which work ok, as well as those over-the-side filters that are fine except for the fact that if you have a tank that has a huge drop between the top of the tank and the water, the noise of splashing can get pretty irritating. I've found the best answer to this dilemma was a pump that can be suction-cupped to the floor of the water half called The Shark Internal Filter. It fits to the floor and does a pretty good job of filtering the water (and quietly too!), and your main concern is making sure that the place where the electric chord (which is well waterproofed and won't electrocute your anuran) doesn't leave a gap so your frog can get out.
Later, I got a really cool set-up for my Firebellied Toads which was a waterfall/river/pond set-up. It came factory ready, and all I had to do was put the pieces together. There are several of these on the market and most provide a sort of through-the-gravel filter, which pumps water through the "landmass" to the waterfall or river mouth and then spouts it back. You don't actually see the filter at all, and the only thing you hear is running water (except when the water level gets a bit low and it starts to sputter air bubbles.) This set-up is really cool looking, not to mention a great filter device and I recommend it to anyone who wants to invest a little cash into setting up their frog-tank to look really nice. I've been posting whatever online links to the filter kit on the Half-and-Half Tank Setup page.
Ask your pet dealer for advice on filters.
One visitor wrote to tell me: "a good way to keep the water clean, while frogs are molting, is to put gold fish in the water. Gold fish eat the skin, and they will eat dead crickets that float around in the water." (This, of course, assumes the frogs won't eat the goldfish!)
Cleaning your tank also falls under the same categories as proper care of fish. Special gravel vacuum cleaners can be bought as well, though they basically consist of a "nice" plastic tube which can be easily made at home. In the best case scenario, you'll be filling the tank with stale water when you clean it. By this I mean, water that has been sitting out for at least 24 hours, AND has been dechlorinated. (You can buy chlorine treatment drops at your local pet store.) That way you are less likely to poison the frogs during regular maintenance.

It is strongly recommended that you check out some of the books in the Recommended Reading list for more info.

Links:

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