Just back yesterday with our 4 year old princess who spent the week swooning over Disney's princesses. Here are my tips:
Princess Storybook Breakfast- Lots of princesses (any Mary Poppins) with a reasonable meal that counted as only 1 TS on the dining plan.
Cinderella's Royal Table- This is a hard reservation to get and I was pretty disgruntled by the cost when we made the reservation. However, it did turn out to be the most "magical" of all of our experiences. My DD4 wore her Cinderella dress. They gave her a magic wand and a wishing star. There was a bit of a wait (Cinderella was doing Cinderellabration when we arrived), but the look on my daughter's face was worth it and the setting was very cool! The food was pretty good too!
Epcot- We found Jasmine, Sleeping Beauty, Bell, and Snow White greeting folks in the World Showcase at Epcot. The lines were very short in teh mid-afternoon and most of the princesses stood in gardens or by the water in spots that were great background for pictures. My DD was able to spend a lot time talking to each princess...They weren't as rushed as they were at breakfast.
I second the Princess autograph books (available at shops all over the World).
Let them wear their princess outfits. My DD wore one to each breakfast and wouldn't take them off. I did bring inexpensive Wal-mart versions which worked out well as she did put her foot through the netting on one dress when she was climbing in the Boneyard at Animal Kingdom. I might have been bothered more if we had paid 60. (the going rate for princess dresses at WDW shops). Anyway, CM's bowed to her, gave her an extra merry-go-round ride, etc.
Both Magic Kingdom parades feature princesses. We happened to be sitting on the curb near the beginning of Main Street and she was really able to get a good view and call out to the princesses. Snow White blew her a kiss and, because we were so close, many of the princesses singled her out for a wave or a handshake.
The whole trip was about princesses for us...and my daughter really had a great time...and we had a great time watching her!