Take out the battery, sim card, sd card if you have one, leave the back off and put it in a ziplock bag full of uncooked rice overnight with the battery. The rice pulls moisture out of the phone. The phone probably won't work 100% but it usually fixes it enough to at least turn it on and retrieve data.
Current Phone: Samsung Galaxy S III
Current ROM: SlimKat
Previous Phone: Samsung Droid Charge (sold)