You push the left analog about halfway forward to gain speed and keep it in that position, don't move it up or down any more except if you need to bump or stoppie, krail, etc. I have to push mine about 1/4 or 1.5/4 forward to get the "optimal" speed in VC, but in SA I have to keep it at like 2/10.
Practice with using the gamepad often and you'll get used to it.