This has already been stated wonderfully before, but I wanted to reiterate...
1) Show up early, before Park "Official" opening. Ask a ticket cast member if they know where the signup is, in case it's been moved. If they don't know, head to the old Sounds Dangerous building. They used to do signup outside at the queue there, but last year (and probably currently) they do it inside the old sound works exit for the attraction, so look there. You may or may not see the line from outdoors.
If you aren't familiar with the entrance (used to be exit) that I'm referring to, it's where the barber shop sound booths and the foley art stations (where you could clip clop using coconuts to Ichabod Crane) were.
2) Every child you plan to sign up MUST MUST MUST be physically present. So, you can't go with one kid, and try and sign up both. They won't let you.
3) Show up about 10 - 15 minutes early for your showtime. They do some preshow prepping for the kids. Sabers and attire will be loaned from Disney, so be sure your kid is aware that their personal light saber and / or Jedi attire should stay at the hotel.
4) It's free (I assumed you knew this already, but I hadn't seen anyone else mention it, so I thought I'd add that)
5) If it rains for your show time, NEVER FEAR. They still do it, just inside the Star Tours building. So, again, be sure you show up and are there early. One year it rained and I assumed it was cancelled. I could never find the group (since I was cutting it very close), but they'd already assembled and gone inside. They also let the group cut into the Fast Pass line after the show from INSIDE Star Tours, which was neat (and that's how I found out they were still doing the show, just from inside.)
So, again, if it rains, don't give up.
It was a ton of fun. My daughter loved it!