the language in which improv is described can often sound new-agey and ephemeral - not at all like the prescribed way in which i hear music theory itself explained.
i feel i can offer a little advice and it might seem a bit counter-intuitive, but how about actually trying to switch "off" your thought process? so to add to what chris says scales.. scales.. scales... then forget them..
- I've found that the cerebral, including those nagging doubts in my head, are my worst enemy when it comes to improvising..for me its the holy grail - to be able to play whats needed and to play just one note that stokes me out..to pretend you're in the band that your improvising for might also work
I don't think any musician can expect to improvise without something to improvise to - so getting together with either a CD or better still, a group is important..hope this helps...Ben
PS - also setting up a synergy between your voice and your bass can be useful for phrasing...its not always the complicated stuff that works as I'm sure you know, but using your voice at the same time as playing can work well...it might be that there comes a point where your bass becomes your "voice"..so the time between intent and execution becomes that much shorter.