My DD9 has been picked twice so far and we have only tried twice.

First time was Jan. 7th. She was the last kid picked. I just kept telling her to jump up and down. (since then I've been told by a CM that they pick the "high energy" kids.) I think she was jumping and waving her hand for a good five minutes. She was so happy to be picked.

She was the last kid jumping up and down, by the way. She battled Darth Vadar. She's only about 60 lbs., petite, wear glasses, and stands about 4'1". She was wearing black and white. They picked 30 kids that day. It was a Wed. and the show was pretty busy. It was the 12:30 one (lunch time) Here's a few pictures from that show.
Second time she got picked again.

We went to the 1st show. There weren't many kids there. They only seemed to pick about 15, or so. They were actually picking older and younger kids than normal to get enough kids. One girl was 14 and another was a very small 4. (they asked their ages) This time she fought Dark Maul. I actually wanted her to fight Darth since one of my fav. PP photogs was shooting Darth but it didn't work out. She was wearing a cape (one I made her to be Hermoine Granger for Halloween-she had to take it off to fight) This was Jan. 28th, again a Wed. 10:30 a.m. Here are a few from that show.
Dark Maul and Darth were at both shows. I think they based the amount chosen on how many kids were there. There weren't 30 kids to pick from on the 28th. I didn't see any signs at either show but we were there during a slower time. If you are there during busy season maybe a sign would help.
Based on our experiences, I would try to get there for the first show, before the parks get busy, mid week is good. Have your child jump up and down as much as possible, waving their hands-tell them not to stop.

They identify them by what they are wearing so bright colors are a little easier to spot, or a funny hat, or glasses, or????
FYI, for the parents, if you are seated on the floor behind the blue line, especially on to the right of the main aisle (facing the stage) be prepared for Dark Maul to get in your face. He did this to us both times-he's pretty impressive!
One more thing, getting a table early won't get your kid picked. They pick form the kids on the floor area which is about 4 feet below the table/seating area. The open up the floor prior to the show for kids and parents to seat/stand in. Once a kid is chosen they go stand on one of the blue circles in front of the stage. Once they have a kid for each circle, they are done picking.
Look away if you are scared easily!