+1 to previous comments. Also, it's good to know where the funky notes are.
Many funk tunes vamp on 7th chords, because, well, they're funky! G7, for example, incorporates the following notes: G (root), B (major third), D (perfect fifth) and F (minor seventh). Approaching the third, fifth, or seventh from a half step below can sound VERY funky. In other words, Bb to B, Db to D, and E to F. Try working out some ideas using this concept, and try them in several keys.
Also, don't neglect your old friend the blues scale. (In G again for consistency: G, Bb, C, C#, D, F, G). Even though this has a minor third it sounds great under a 7th chord.
Chromaticism works great too. Try going from the root to the sixth below, and walking chromatically back to the root. Play two eighth notes for each note, spaced by an octave as suggested by sammyp.
Also, open string keys such as E or A are very common, because those open strings open up further possibilities. That said, don't rely too much on them. Some guys can only slap in open keys. What I'm trying to say is, learn to slap in all keys, but be conscious of some of the additional possibilities with open keys.
Hope this helps.