What imporvising I do is usually worked out, sorta, in advance. Sounds like a contradiction in terms, but it isn't, really. I tend to use the blues scale quite a bit. In the key of C for instance that would be C,E^b, F, F#, G, B^b, C.

I say "worked out,sorta, in advance" because I find that I don't play the imporvised passage exactly the same way twice, cause I can't remember sometimes how I played it the first time around. At other times, I just slip in a little hot lick that I worked out a long time ago in place of the melody. That I can play the same way time, after time.

For me, the key to imporvisation strange as it may sound is based on my familiarity with the piece. There's all sorts of stuff I can do on Forked Deer or the old hymn What a Friend We Have in Jesus because the melodies and chord progressions are so familiar.
Jim