Ah, memories. Once upon a time, I was the accompanist for my school choir. Ended up doing that for all my years of jr. high school (coz I was cheap labour, the teacher only have to give me a grade! :P)
Anyway, Matt's suggestions are very good. I had to resort to a bit of memorisation myself just to be sure I'm not missing anything. A tiny bit of improvisation doesn't hurt, either. If I didn't have a page turner (which was often), I usually turned up the bottom right corners of the pages and turned, very quickly, with my left hand. (One of the area church Music Directors is very good with this, but sometimes the loud noise that comes from his violently flipping a page is a bit disconcerting ...) Sometimes, I practice a piece incorporating a page turn if needed. I know, that doesn't sound all that clear, sorry about that. If a piece is particularly long, I ask the music director if it's okay to photocopy a piece, then just tape the pages together so I eliminate the page turning problem. Usually, that's not "kosher" as most pieces are protected by copyright law, but as long as you promise to shred and destroy said photocopies, there generally isn't too much of a problem.
Have fun! Accompanying a choir can indeed be a rewarding experience.