Frogs found in nature have a very wide variety in their diet, such as spiders, tadpoles, guppies, and worms. For example, poison frogs lose their toxicity when they are bred in captivity because their diet is no longer as diverse as it would naturally be. Realistically, we can't provide the complete variety that would be best, but keeping this in mind is a good idea. Crickets tend to be the most available food, and generally make the basic staple for frogs. Some frogs require much bigger meals, such as mice. See the More Info page for more resources on what to feed your particular species of frog.
Crickets can, and should, however, be supplemented with special calcium gut pellets, like "Calcium Plus, food for Crickets", that provide the frogs with necessary minerals. These can be found in most pet stores. Drop the pellets in with the crickets several hours before they get fed to the frogs. Then when you feed the crickets to the frogs, they will have the proper nutrients in their systems for the frogs. There are also powder supplements that can be sprinkled on the bugs before you feed them to the frogs.