I believe that some of the greatest basses today didn't have dvd's or study materials but they basically studied other basses and used them as a road map. So probably to further advance is to study from live concerts dvds.
A good dvd to purchase is bass day 98', you can't go wrong with marcus miller stuff cuz this is the closes to the stuff gosepl musicians do. Victor bailey stuff.
What i do is listen to a good track, play it, then see how can i make it sound better with my own improv skills etc... Or i'll listen to some Israel houghton, and think of, how can i make this sound better by improv. Besides dvd, the best way i learn is just playing with a live band, maybe doing a lot of shed sessions. for some reason, my improv and creative skills come out more when i play live for an audience or something is that situation or playing with a band.
But since this topic is recommended dvds,
i would say:
1: bass day 98
2: A night with Stanley clark (bass)
3: victor wooten- bass day
4: kirk whalum