I decided to make a seperate reply for these pix since they're so darn cute! Rosetta was his favorite character to meet. He absolutely loved her!! And no, he's never seen the Tinkerbell movie!
You can just see the "MOM!!!" in his eyes. "I'm talkin with my woman!"
Unfortunately all I can say is keep your eyes open. It takes a lot of work (and luck). Sometimes you need to be in the right place at the right time. Good luck to you!
The biggest "tip" I have is that be on the lookout for Secret doors around buildings. One time we were shopping in Adventureland when all of a sudden a wall opened with Alladin and his handler. We got a quick pic and autograph. If you see a big blank wall near a shop or some other place, just look it over for a door seam.