Wet = plenty of spare everything. And I mean everything.
An air mattress is a very good idea, especially if it's cold. (If you can't get a pump where you're going, self inflatable mattresses are pretty cheap and last a while!) The best way to keep warm is to stay as high off the ground as you can. Remember, a blanket below is worth two on top!
Also, a flashlight is absolutely essential. Whether it's reading a book in the dark or finding the washroom at midnight, a flashlight, even if it's cheap and/or disposable is a must.
Speaking of books, make sure to bring a book, crossword, or journal/sketchpad to keep yourself busy during down time or if it rains. Cards are also a nice idea.
A collapsable chair or a waterproof cusion is always great for meals or campfires. If it's wet and cold, you don't want to sit on the ground, and any benches provided will probably be very cold and uncomfortable.
Avoid bringing electronics, like gameboys or cellphones. They can get lost or ruined way too easily, especially if it's wet. A camera, however, is a good idea to capture all your memories or to simply alleviate boredom. An ipod is sometimes a good idea, like if you have trouble going to sleep, but remember there's a chance of it getting damaged/lost/stolen. And if you can, leave the makeup at home. You won't need it, I promise.
Don't forget the little things, like toothpaste, toothbrush, deoderant, personal items, and a hairbrush and plenty of elastics. By day three, you'll be grateful you can hide your bedhead in a ponytail!
Most important of all is a willingness to have fun! It may not always be the most exciting/clean experience, but roll with it and I promise you'll have at least one good memory!
