If it's a 4 or 4S, I would suggest a
Lifeproof case. They're waterproof up to 6 feet underwater, dirtproof, freezeproof, and shockproof from up to a 10 foot drop.
I'm really happy with mine, but they are about 80 bucks. If you're not constantly exposed to starfishs who will throw your phone into water, or don't have to deal with water, mud, snow, or large drops over 10 feet, I wouldn't recommend it.