I heard that if your child has a Star Wars t-shirt on and/or makes a poster they may pick your child. My son only wears SW T-shirts and made a bright yellow poster that said "Train You Must to Battle Dark Side of Force - Yoda" The Master Jedi laughed and picked him. He had a great experience! - This was in Disneyland tho...good luck!
I suggest getting there early and being in the front. They usually take kids up front and if you don't get in and have time, go to another show. They might remember your kids from the first show. I don't know if a sign is necessary, but having them wear Star Wars apparel certainly shows that they are fans and want to do this more than the girl in the Princess shirt who is raising her hand just because (she will be there... she's always there...).
Before you embark on trying to get him picked for the Jedi training I suggest you ask yourself objectively, how will he/she feel or act if they don't get picked. Some kids don't deal with disappointment very well and not getting picked can be such a big deal to them that their whole day can be ruined by it. So if you have a child that falls into that category I would suggest you might be better off not even trying and at the very least don't tell them about it because if you do tell them about it they will get their hopes up more than you realize even if you tell them they might not get picked. In the typical kid's mind they will feel certain that they will be picked.
Now that said... you need to make sure your kids will act like he wants to be picked. If they are shy then the odds are they wont be picked because the person picking them wants kids that will be active because it makes their job easier.
Second, lots of kids wear the star wars t-shirts because lots of kids are doing exactly what you are doing and trying to game the system... so if you really want to be picked you might need to take it up a notch... as in having them wearing a jedi robe or something (if you are going in October it will be close to halloween so robes should be plentiful).
And by all means be sure you get them in front center or slightly to the right... you can also make sure you stand behind them to serve as a block to the mass off other kids that will rush forward trying to get picked (yeah it might sound naughty but how much do you want to get it... that is the question... and this is a zero sum game if your kid gets picked some other parents kid doesn't).
Good luck... and if he doesn't get picked there is always the pirate tutorial at Magic Kingdom.
My son has had the extreme fortune of being trained as a Jedi at both WDW and DL. Both times he wasn't wearing a SW shirt, we just got there early, got close to the stage, and he just acted excited. I will say that I enjoyed the WDW one better. DL had two villians, twice as many participants, and it seemed more rushed.
My one piece of advice, have your camera ready for the training. You can get some great pictures when they are going through their training exercises.