[Goldeye]

Tuning this year's PD system (our first as a team) we were able to get it to run ok with wheels in the air (and then for some time before further modifications) by starting with a P gain too low than tuning it up till the oscillations just begin..
Keep in mind that the weight and friction of the robot acts as a serious D controller, which does prevent major oscillation.
I would just tune the P gain until you don't get a lot of major oscillation and then see how it works on the floor.