Learn chords in all keys. It is not enough to know the bass notes -- to improvise successfully, you must know all the notes in a chord and the chord's function, so that you can successfully substitute other chords or harmonies during your improvisation.
Practice approaching new chords from a half step above or below. This can help you create the walking bass line so common in jazz.
Listen to the other musicians in your band. You can get ideas for improvisation from them. If you hear one of them play an interesting rhythmic line, try to imitate and change it -- without losing the pulse of the music.
Transcribe the solos of great bass players such as Charles Mingus, Oscar Pettiford and Ray Brown. Listening to what other players do will help improve your improvisation skills.
Practice scales and arpeggios other than sevenths, majors and minors. Some useful practice scales and arpeggios are modes, pentatonic scales, blues scales, diminished and half-diminished arpeggios, and chromatic and whole-tone scales. You can use these scales, or elements of them, in your improvisation.