Keep an eye out on the sides of the castle as you are walking in. We stumbled upon Cinderella, Sleeping Beauty, Ariel, and Snow White. You can ask a cast member and they can check what times they will be there. If you go to the princess show, you can see all of the princesses from a distance. I have heard that they sometimes come out after the clean up so you could try sticking around. There was also a princess story time in the same area.
When you wait in the huge line, you only get to see three princesses and you never know who will be there - they rotate in and out. If you are having a hard time finding the princesses, everyone says go to Ariel's Grotto. We hit the princess jackpot last time we were there and ran into almost every princess around the park so we never went.