Foil is easy to use to cook over the campfire. It is light-weight, inexpensive, widely available, can be used to cook a variety of meals, and cleanup is extremely easy. Just roll the foil up and toss in the garbage. You can even eat right out of the foil packet if you want. Here are some tips for using foil to cook campfire meals:

Use heavy-duty foil. While a bit more expensive, the extra thickness will help keep your food from burning and will conduct the heat better. Two layers are best. While a common suggestion is to put the shiny side of the foil to the inside of the packet (to reflect heat better, I suppose), Reynolds says that it doesn't matter which side of the aluminum foil you use. Both sides do the same job. Even so, I have always put the shiny side in and I suspect most of you do the same.
Foil cooking depends on moisture inside the foil packet, so make sure that the folds of your foil packet are tight and that the food you are cooking has sufficient moisture. If not, add some by folding the packet up except for one end, and then add a tablespoon or two of water, broth, salad dressing, or butter before you seal the packet. Sliced onions can also be used to add moisture.
Cooking time will vary depending on the temperature of the coals. Make sure to cook your meal sufficiently. Check one packet for doneness before pulling all of them out of the fire. That way you will only have to rewrap up one packet if they still need more time. Remember that some vegetables (potatoes and carrots, for example), may take more time to cook through.
You can also put newspaper between the layers of aluminum foil to help keep the food from burning.
For quick foil meals, microwave the food before wrapping in aluminum foil. Then when you get to camp, you only have to warm up the foil packets instead of cooking them completely, which will shorten your cooking time considerably.
If you need to warm something up, wrap it in foil and put it near the edge of the fire, turning occasionally (I have warmed up tortillas for burritos this way, for example).