If there is a character you are looking for you can ask at city hall. They'll be able to tell you when and where most characters are. Then there are some that you'll find at random throughout the park.
Mickey and Minnie can always be found in toontown. Pooh and the gang can always be found in critter country. They have a set area. If you are looking for a princess try fantasy faire in the theater between fantasyland and toon town. They used to be around the castle, but from what I've heard they aren't there now that fantasy faire has started. There are usually character near the maingates in the morning. There are often characters outside the hat shop on mainstreet, near the theater where the 50th movie/ Mr. Lincoln are. You'll often find a few roaming around fantasyland. Buzz can be found in tomorrow land and Woody near the golden horseshoe.
The best time of day to find characters is first thing in the morning until early afternoon. From about an hour before the parade until after fireworks and F! you wont see them out and about.
