Strumming a few bars is good...I always try to throw some little improvised stuff in, like suspended chord stuff, adding 9ths or major 7ths, descending or ascending bass notes to the chords. Most every song I play has some little minitune going on in the intro and breaks, also know as a "riff" (shock horror!). I make them up when I first play the tune, and they kinda stick in my head for whenever I play it again--therefor continuing the folk process!

--Mbo