I try to stick to a theme I'm working on. It's usually my boys but I still have specific paper picked out for the various pages I'll do. My biggest downfall is cardstock. I suppose I could find coordinating colors for all my patterned paper but that just takes some of the fun out of it. So I usually cart around my cardstock.
That and I always bring stuff for cards. If I happen to run out of inspiration or photos, I can always make a few cards to stretch the noggin.