As far as my improvisational skills go... They're limited by my lack of speed and clutziness hehe. But... I used to regularly jam at the local Blues nights, both guitar and bass, I used to be much better.
In a rock/
band situation, my solos were still mostly improvised... I'd have an idea that I liked perhaps from the take before in practice, and just build on that... As far as guitar goes... "Why?" and "I Can't Pretend" had what were originally improvised solos... "I Can't Pretend" ended up sticking as a regular solo, and "Why?" i still kept ideas for certain parts of the solo, but improv'd the rest... i do that a lot...
When I'm playing bass, i end up improv'ing a lot more, oddly enough... "You" is basically one big improv for me. Of course, after recording it I solidified it as having real solo parts and the verses were still off-the-cuff, buuut... That was written basically before hitting the studio (as was "I Can't Pretend"). "Dark Angel" and "Conscience" (Goddamn I love that song!) were totally improv for me, every time I played them i played almost all of the solos differently, and they rather sucked. The recordings, however, were nearly completely improvised in the studio. Usually when recording, I'll just do my favorite thing... Fuck around until something sounds non-crappy. Two or three takes later, got a kickass solo that stuck around as a real solo.
Now that I read this, I don't know if it's really important... I'm just rambling. But I wrote it so it's getting posted. hehe.
I guess I don't have a real method of improvising... My theory has basically gone out the window while being in a band for two years. Some stuck, but most went byebye. I just kind of started playing with my heart more than my head. In other words...
I just fuck around until something sounds non-crappy. ^_^