When we did the Jedi Training Academy, there were a few kids that came with signs, and they seemed to get picked. Getting there early doesn't seem to help much, when I took DD, we were among the first there, and we didn't get picked. Mostly I think you just have to make sure that your DS is kind of out there when they are picking kids - when we went, they asked all of the kids to jump up and down and shout and wave their arms so that the Jedi Master would pick them.
You can also go back and try again if DS doesn't get picked the first time - and depending how old DS is, make sure he is prepared just in case he didn't get picked - when DD didn't get picked, she started crying and we had to leave, which surprised me, because I didn't think it was that big of a deal to her.