Practice the modes and scales to get the foundation. Also, you can isolate the chords of the passage you want to solo to, then break the chords up note by note (an arpeggio) and find ways to get between the different notes of the chord or chords.
If you want to improvise solos rather than just write them and perform them note-for-note, you just have to start taking chances. If you have a cheap keyboard you can have it play some background beats with chords (however cheesy they may be) and just go crazy taking chances and finding out what works and doesn't work. Improvisors (soloists, dancers, free-style rappers, stand-up comics, politicians, etc.)have to be convincing more than anything else. So when you play a "sour" note, it's like falling in a hole. You have to find a way to get out so that no one notices you fell in the hole-- convince them that you intended to play the note by tieing it together with some more notes afterwards that complete a statement or emotion. It's like telling a lie and realizing your story has a hole in it, you've got to find a way to fix it and still be convincing. You'll never be able to rehearse everything, so you have to learn how to make it up as you go. Kick ass and have a blast!