Be there early!
Arrive at least a half hour early so that you can sit down right away as close to the line as possible. You need to be in the front if you want to be seen. There is a taped line on the ground, sit as close to it as possible. If you want pictures of your child (and I'm sure you do) then you need to arrive an hour before show time to grab a good spot to see everything. You can sit in the Tomorrowland Terrace Cafe area against the railings for the best picture spots. Anything against the railings is a good spot to grab pictures.
Holding up signs
Hold up a colorful, noticeable sign that says "pick me." It doesn't have to be any bigger than an 8.5 x 11 but put it on bright colored paper with bold lettering if you can.
With siblings it would be cute to hold up signs opposite each that say,
"pick my brother --->" or " "<---- pick my sister"
"pick him --->" or "<----pick her"
Make sure your child is paying attention.
Stand up, jump up and down!
Scream "Me Me Me Me!!"
Wear something bright and noticeable.
They usually do call on two siblings if they are right next to each other, dress them alike to increase your chances.
If you do all of the above or at least some of the above you will most likely get picked. Don't be shy, act willing and your chances are good. They want kids that are willing and charismatic.