I was there Sunday 8/19. It seemed to me that the more excited your child is, the better. It can't hurt to dress up in character, but don't think that was necessary. My son was very excited, jumping up and down and yelling "pick me, pick me", and then I started to also point toward him (from behind, over his head), and yell "pick him, pick him", and at that point he was selected. They pick a lot of kids, and if your child is in the front row and energetic, I think you have a good chance that they will get chosen. There wasn't one specific location that seemed to be picked more than others. I've read about parents making signs for their kids (e.g., "The force is strong in this one!"), but I didn't see any of that, and I don't think it would be necessary.
Most important is to make sure your child knows that they might not be selected (avoid the disappointment), and you may need to be willing to go to more than one show if they are determined to be selected.
-Bret