| I think playing off ones self comes from studying the concept of motivic developement. be it rhythim, harmonic, or melodic, there are just so many places you can go to with very little to start with.
as far as being regimented in my practice of that.... I think I was to begin with when i was first working with the concept of developing ideas through a solo, but it's definitely become more of a sub concious thing as time has worn on. I will often set up a loop when I'm shedding, and then just pick two or three notes with a rhythmic pattern to start playing off of. I can normally sit there for hours developing something that started off as just a whole step interval, and then becomes some minutes or even hours of music when I'm shedding. Of course this is all condensed down into nothing more than a couple of minutes when you take a solo on a gig, but having the flexibility to be able to do that for hours without getting bored is a strong improvisational tool.
I normally start off with an idea I've heard in a solo on record. Especially when I don't have time to transcribe an entire solo, and I just want to develope a specific idea that caught my ear. I'll take anything from a two note motif to a three bar phrase, and then work whatever idea it is across my whole instrument in as many different musical contexts as I can think of. And then a great bi-product of motific developement is that you come up with phrases, shapes, colours, and sounds that become a part of your own sound, increasing your language and increasing what you have to say when improvising. And then of course it's all tied into the intensive study you mentioned in the end of your post. That fundamental aspect of language be it in music or speech, comes from years of study. And those more fluent and elequent sounding when they speak are those people that have put in the time to study. There's really just no way around it.
Easy,
Janek | 
Practicing Improvisation 
