Bumping this to add my experience.
I read this entire thread before coming to DL. I read repeatedly in the thread that "I've never heard of a kid with a sign not getting picked" so I felt confident about our chances. We followed the tips given (bright orange shirt, clear star-warsy sign, lots of jumping and controlled enthusiasm). My son had his heart set on doing this particular thing.
We ended up spending the entire day in Tomorrowland and not seeing any of the rest of the park, because he kept getting passed up despite there not being many signs, costumes, or star wars shirts in the audience. By the third show, he was so incredibly sad and told the kids next to him "I never get picked" that it just broke my heart. He finally got chosen for the last show and was hugely excited, but I feel it's important for parents to realize that "having a sign will get your kid picked" (kind of the tone of the thread) won't necessarily work and ultimately you may have to deal with some major disappointment and wasted hours. The show is fun even if your kid isn't in it, so it's worth going, but temper expectations.
The Jedi master CM today seemed to pick primarily from one side on the audience (we sat on the wrong side twice) and there was no apparent rhyme or reason to his choices. There were handicapped kids in each audience and they obviously got picked, but otherwise, it was a total crapshoot. I think we lucked out at the last showing just because it was a schedule conflict with the parade and there wasn't half the audience they had at other times.
I would recommend skipping the parade in the afternoon if you want to maximize your chances of getting your child in, if they really have their heart set on it. The show is entertaining the first time, but by the fourth viewing in one day, all the jokes were groan-worthy.
