1. Take your child to the school a couple of times before the start of the school year. Walk them around the halls. Let the feel the walls, adjust to the lighting, and get used to the pattern of the tile on the floor. This might be a good time to map out their routes, if they will be changing classrooms. Show them where the restrooms are in relation to their class room/rooms. Let them see their classroom and become familiar with it as well. I understand you can't do this to far in advance, but the more visits you can get in the better.
2. Introduce yourself and your child to the teacher. Communication between you and your child's teacher through out the school year is going to be key in their success. You might as well make your self known right away, and let then know you are an involved parent. Let the teacher know you are their to help however you can. During this time your child will be able to become familiar with the teacher as well. This should help the transition if there is a recognizable face among all the craziness.
3. Rehearse, Rehearse, Rehearse! Devote a little time each day to going over scenarios that your child might encounter while they are at school. What to do when you get upset at a student/teacher is a good one. What do you do if you get lost trying to find your room. Who do you go to if someone is picking on you or bullying you. For that matter make sure your child knows what bullying is and understands that it is not OK.
This is just a general guide. You know your child, if preparing them ahead of time will only make them perseverate on it, then this will not work for you. I hope this has been a help[ to some.
I have more resources on my website at http://autismspectrumresources.blogspot.com