I use a Iphone Otterbox defender case. It has a plastic cover for the whole phone plus a cover for the glass. Then there is a rubber cover that goes over that. If you are worried about moisture get the PDA Armor case.
http://www.otterbox.com/apple-iphone-4-cases/apple-iphone-4-cases,default,sc.html?cid=MSAPIPHT10&utm_source=msn&utm_medium=ppc&utm_campaign=otterbox-iphone