Like Azrael said, practice alone with songs.

What I did to be able to improvise a solo was to go on the net and find some midi files I liked. I love playing with midi since you can take some instruments off the track and replace them with yours.

Helped me a lot, but the best advice I can give you is not to be afraid to try things. Try to put in some licks you've learned, even if you mess up. With time you'll develop timing and you'll be able to play what you hear in your head.

Normally I never listen to those voices in my head but with improv it's ok :)