If you haven't already, take her to visit the new school and meet her new teachers and classmates. Any preschool should be more than happy to let you hang out in the room for 15-30 minutes so she can see what's happening in there and check out the new toys, etc. Try to go in the morning, when all the kids are there and are having "circle time" or some other fun activity. Seeing the classroom in the dark at naptime or while everyone is eating lunch is not a lot of fun for a kid.
Talk to her about how much fun she will have at the new school and the things she will do and learn ("You're so lucky that you have that train in your new classroom, that big slide on the playground, you get to eat lunch on the red table, sing that fun song", etc.) If she needs anything new for the new school, talk to her about how much fun it will be to use the new lunch box, nap mat, etc.
Beyond all this, just relax and know that you've made the best decision for her. Stay upbeat about the new school, and make sure she understands that you will be back later. My daughter started a new preschool last year after being in the same day care since infancy. On the first day, I was prepared for tears (from both of us) and clinginess, but she was so excited when we got there she ran off and shouted "BYE, Mom!" from across the room. I had my own personal breakdown in the car, but I felt a lot better knowing that she was comfortable there and was having fun.