We moved overseas and back with our kids...of course in the middle of the school year. (The last move found son in 2nd grade; daughter in 1st) One thing that really helped when we moved was to get into contact with the "new" teacher. Because both kids knew the name of their new teacher, we could talk about what "Ms. Teacher" would like to see, etc. We were able to e-mail the teachers, too, and they were able to tell us something about their classes ("You will be the second Sarah in our class this year!" or "There are 13 boys and 11 girls in our class"...). Just knowing SOMETHING about the new class and school helps prepare the child for what he'll see.
Another thing that helped with the transition was for the kids to bring something from our old home to give to our new classmates. We brought a local candy (peanut free!) to hand out to the new classmates. We had to check with school policy to make sure it was ok, and both teachers were thrilled because it gave the new students something to immediately connect with my kids. This also gave our kids something to look forward to sharing about their old home with their new classmates.
My daughter is incredibly shy, especially around new people. Today she tells me she's good at making new friends. Although I don't see that she is especially outgoing, apparently she is comfortable with being around new people now...and knowing my child is comfortable is a wonderful thing.
One other thing that helped our kids with the transition was that we were able to take pictures of the new school to show them. We'd gone ahead to find a place to live, and while in the new location, we snapped pictures of the new school. That helped, too, in knowing what they'd see. The school they go to now has on-line pictures, so if you know where they'll be attending, maybe your new school will have the same thing.
One more thought is to talk to your child about the move. We talked until we were talked out about the new school. Sometimes the kids would participate in the conversation, sometimes we were simply planting seeds. "Your new principal will be a man." or "I wonder how many boys will be in your class?" or "Do you think you'll have recess before lunch or after lunch?" Some of these thoughts helps the child see that every school is alike (all have recess, principals, etc), but also differences (numbers in classes, etc). Sometimes we got answers to the questions before they started, sometimes not. Always, I let the kids know that I was there for them no matter what....and they've transitioned wonderfully!
Good luck with your moves and transitions!