My DS first attended overnight summer camp when he was 8 y.o, but he wanted to go so there weren't too many jitters about the whole thing. We explained what he could expect - sleeping in the cabin, communal meals in the dining hall, using the camp bathroom etc. It helped that his friend was also at the same camp, so they thought it was a great adventure together and were pretty excited before they went. This boy had been to camp before and he was familiar with the routines, which was nice for DS. DD was about the same age when she attended an overnight camp, possibly a little older. Again she went with a good friend who had already attended camp, and they were excited about it.
They both had done a lot of sleepovers by the time they went to camp. They liked using sleeping bags, so that wasn't a problem. I made up a little special "camp kit" for them with a camera, a flashlight, some basic first aid (really just fun character band-aids, insect repellant and sunscreen), a book, some cat's cradle string and a deck of cards and a few other fun little sundries. For familiarity and comfort, they took their own pillows and in DD's case, a favourite stuffed animal. We limited the snacks, but some kids probably packed more candy and chips than socks and underwear.
Practically speaking, I'd make sure she's familiar and comfortable with using a sleeping bag. If she hasn't done any sleepovers, I'd have a couple of trial runs so that this isn't her first overnight without you.
I'd find out how the students are sorted into the cabins. Can she pick her cabinmates or do the teachers decide? If there are any bullying or social exclusion issues at school, I'd want to deal with them now so that they aren't festering when the kids go away. Last year, DD was on a 5 day-4 night trip out of town with her class and she knew that there would be "issues" with one of the girls in her hotel room. There were 4 girls to a room. One of the girls is a drama queen, for want of a better term (and a psychological diagnosis that I am sure could apply). The drama started early, long before the trip (would she be in their room or another, who were her BFFs, did she get fair treatment from everyone.....) We talked a lot about managing around this girl, how to deal with the drama etc., but it was an ongoing conversation we'd been having for the entire school year.
I'd find out the camp policy on calling home so that you are both clear on the rules ahead of time. That way there are no surprises if she decides she wants to call you but is denied because campers aren't allowed to use the phone (or their own cell phones).
Cell phones, handheld electronics (Nintendo DS etc.), are often banned on these trips, so if your DD is a heavy user, I'd also deal with that up front quickly.
How do the other kids feel about it? Usually there's excitement up front, but a lot of kids are feeling a little nervous too. You can talk to her about the difference between excited anticipation and anxious worrying about a new situation. Both feelings are normal, and can exist at the same time. She wants excited anticipation to win out over anxious worrying! Ask her what she thinks will make her comfortable and happy on the trip - she may have her own ideas. If she doesn't already have one, a little digital camera is a great idea so she can record the trip and may help focus on the excited anticipation rather than the worrying.
Hope she has a terrific time!