A six wheel drive can be done one of two ways (well, these are the two most basic ways to do it).
The most common and in my opinion best way is to use 6 high traction wheels, with the center wheel mounted a small amount lower than the others (1/8" or so).
AndyMark Plactions are a good combination of strength, weight, and price; my team was satisfied with these wheels last year. You can also buy AM's Performance line, which are aluminium instead of plastic, but at a higher cost. IFI
also sells wheels that work pretty well.
Optionally, you can buy omni wheels and use them for the outer four wheels in your six wheel drivetrain. This makes turning easier but also makes it easier for opponents to spin you. Omni wheels also have less traction in the forward direction than a normal wheel.