Have all the music down cold beforehand. They'll see that you've put a lot of effort into it, and they'll have something to tech you on. Also, be able to adapt to changes fast. They throw a lot of stuff at you to see if you can pick it up quick enough. They need you to be able to do this, because the summer is fast paced and the show will always be changing based off of the judging and rankings.
Hope it helps.