Peter T. I think of improvising, in the most general sense, as playing single note phrases during a solo. The pentatonic scale is a great basis for blues/folk improvisation. Here's a couple of tricks I've picked up to help make it easier.

First, I'm very visual, and early on someone showed me the PATTERN of the pentatonic notes on the neck. I've heard this called "playing the box". If you figure out the pattern for E (i.e. where the notes of the pentatonic scale are in relation to to the E string) you'll be able to transpose the same pattern for any key.

Second, for those good bluesy notes, the best note to bend is the 4 of the scale. Just push that string up listen to her moan.

(Forgive me (a) if I'm repeating something someone's already suggested - I tend to skim over the long threads, and (b) for being so doggoned inarticulate about music theory)