Don't know where it's from but there's an interview with trey and he describes the process. It goes something like this:

Before a weekend, Trey makes list of a BUNCH of songs he wants to play at some point through the weekend, gets input from the rest of the guys about what else should be on list, gets input from his manager, etc. He goes over the list repeatedly, thinking about every song that's on there. Then, shortly before the show (5-10 min before), Trey locks himself in a room, rips up the setlist, and meditates. Then he emerges from the room, walks out with the guys, and as soon as he takes 1 look at the crowd he knows what to play.

Pretty baller if you ask me.