Play scales in 3rds, 4th, 5th, etc... 3rds for C7 for ex would be C, E, D, F, E, G, F, A, G, Bflat, A, C, Bflat, D, C. (hope I got that right
Or better yet, learn to play the appegios of chords. Play the root, 3rd, 5th, 7th, 9th, 11th, of a G7 for example. Then do this from all different starting points, all over the fretboard. sometimes use all the strings, sometime do it on only one string. Vary when and how you shift and use different fingerings. Do this for all the types chords (major, minor, 7ths, diminished, augmented etc...). So start with Cmajor, then do Cminor, etc..., the do D major, D minor. I think you get the picture?
Any finally, the best thing to do is to transcribe other solo's. You'll be amazed what you learn. and don't transcribe only bass solos. Find a Miles Davis solo that you can learn by ear (or read the music) and apply it to the bass. This will help yo play more melodically -which is the point of soloing (improvising a melody)
