Back in Philly, we would just get him a bit drunk, stuff him in the back of somebody's Tercel(with instruments) and take him to Ortlieb's. where half of the audience were trombone players, waiting all nite, horn in hand, for a chance to play a couple of choruses of Satin Doll--

Anyway, I get a feeling that PeterT (and most people, for that matter) get "improvising" confused with "following". Following simply being a situation where you don't know how to play the song, but get through by ear, using whatever cheap tricks you might have learned--"improvising" is creating a part or a solo extemporaneously--

The key here is that you can and should know how to play the songs that you plan to "improvise" on "inside-out". I have in fact, had people (real jazz musicians--some of whom actually knew Bill Cosby) sit out tunes that they did not know--

As to ettiquette--improvisation is a group effort--the ground rules are generally layed out at the beginning (choice of tune, key, tempo, what the vamp for soloing will be--how the soloing will end, and what the end will be, and can extend to what the riff that the players work with will be and what kind of scales they use--on into the nite)--depends on the personalities, tastes, and skill of the players. The better the player, the easier they are to play with (if they want to be!) The thing is, when you play, communication is the key--it should be a group effort--when it isn't, its a mess!

Also keep this in mind--it is way harder to play the melody than to create a solo that fits over the chords-