Meeting Stitch is very, very, very tricky business. Your best bet is to just hang around in Tomorrowland around his first set and catch him as soon as you see him. From my understanding, the way "roaming characters" work is that they'll greet two or three guests who walk up to them, but as soon as a line starts to form, they walk away to a new location. So if there's already a group of people around him when you see him, hang back and wait for him to finish up because he'll leave and walk to a new location if too many people are approaching him. Last week I saw six or seven people with him in front of the Peoplemover. I turned around to get my autograph book out of my bag, and when I had turned around, Stitch was walking away and the crowd had dispersed.
Now, I'm not an inside expert on elusive roaming characters. If what I'm saying is wrong, I apologize. If there's someone out there with a better understanding on how roaming works, please correct me!