I play a 12 (triple 4). I just use very strong and deliberate movements with my left hand. My left hand technique is pretty accurate to begin with, so even moving to fretless was of little problem, and thus the 12-ver presented little challenge in terms of technique - but it does require added strength and diligence in articulation. It's a bit of a workout.
I can actually play it fast enough to develop enough forearm pump to actually put me out of breath. I also find that I can play fingerstyle, picked, and even slapped...but again, I have played many other basses with both wide and narrow spacings, so I guess I adapt well.
Just keep at it, and try to be as deliberate with your fingering as possible.
Although not mixed too well in order to spotlight the 12, the title track from Michael E. Thomas' last record features me on 12-ver (I actually used it for 3 cuts total).
http://www.myspace.com/rubydust
Check out the cut "Wreckless Abandon". The 12-ver is most obvious on the octave glissandos.