And here's another one I just thought about, Victor Wooten said something about it in Bass Player a lot of years ago, and I got some good out of it as well.
Play with a metronome. Start with it set at quarter notes (1,2,3,4), and use a moderately complicated bassline you're already comfortable with, or freeform if you like. Play for a while. Halve the time on the metronome, and play the same things while the metronome hits on 1 and 3. Then switch over so it's on 2 and 4. Halve the time again, and play for a while with the metronome hitting on each beat (1, then 2, etc). Halve it again, and have it play on every other measure. It's quite a workout to play something complicated, and keep good enough rhythm that you land on the 3 of every other measure right when you hear the click, but it does wonders for your time!