This is a real toughie for me, as I have only begun to improv. in the last year or so. Granted, before that I seldom played the tune exactly the same twice, esp. with Irish ornamentation, but I'm not sure that is true improv. The band has finally started playing a little more blues, and blues-y folk so I have dredged up some theory and started using diatonic scales and slides. Still not very good at it, but once in a while "Frankie and Johnny" comes out AWESOME!