The best ideas have already been said:
Learn to hear chord changes and predict them.
Watch other guitarists' hands. (Pick a GOOD guitarist!)

If you can tape a session and practice at home, it would be good. Even if you practice with pros' CDs or tapes, you'll get used to hearing and predicting, as well as maintaining tempo, and they won't throw things at you.

A friend of mine Ryan Thomson ("Captain Fiddle") has written a book: Chord Encyclopedia, with chords for 1400 fiddle tunes. (Irish, American, English, Canadian, Scottish, etc.) I was visiting Rick Fielding and he was talking about accompanying other musicians. He whipped out the book, and said "You NEED to get this!" I looked at it, and found it was written by Ryan, whom I know and who lives less than 10 miles from me and is the only person I've ever taken 'official' lessons (fiddle) from. It's a good book, and he takes orders from everywhere.