I'm a flat picker - and cross picking is a technique that uses a flat pick. All of the folks that Rick mentioned are the creme de la creme of this style. I got most of my ideas on this from Doc Watson, Dan Crary, Happy Traum, Jerry Silverman, and Norman Blake (who IMHO is the best). It is, according to these guys, a down up pick movement - but what strings are struck depends on the tune. And there are two major sequences of it and derivitives come from these basic styles. I call it idling - when not pursuing an active lead/melody line crosspicking makes a great filler and sets up the song for the next straight run of notes.

Bluegrass Guitar by Happy Traum
The Songs of Doc watson (Oak Publications)
Flat-pick Country Guitar by Happy Traum
The Flatpicker's Guitar Guide by Jerry Silverman

All describe exactly the same styles as the basics. After that it gets real unique!! And fun *BG*

Steve