I bought a case for my iPhone 4s (but they make them for iPhone 5 too) called "Lifeproof." I have to tell you, it is AWESOME!
I'm normally very frugal and would never consider spending what I spent on this case, but I figured it was an investment as I don't invest in phone insurance. I thought of it as a pre-paid insurance plan on an expensive phone!
The case is waterproof, dirt proof, drop proof, etc. I've had it for a few months and so far I've dropped my phone in the toilet (fully submurged) and slammed it in my car door when I was wearing a long sweater and it was hanging out of the car door. When I dropped it in the toilet was the first time I got to see if it REALLY was waterproof and I was skeptical! However, when I took the case apart, the phone did not have one drop of water on it! I also let my kids (especially 6 year old son) play on it ALL THE TIME and don't worry about if he's going to spill a drink or anything. I do have to wipe the screen because he manages to get it sticky, of course!
I paid $80 at the Sprint store for the case.

BUT...you can find them on Ebay or other places online. I know someone who said she got one online for $40, but when I held/felt hers...I questioned its authenticity because it feels different.
It is really light weight and just awesome all around. I highly recommend it!