Some previous discussion on this thread.

The way I learned was to keep the finger in place on the fingerboard and move the hand so the finger rolls from side to side. Imagine you're being fingerprinted. You want to make sure you get the sides of your finger, so you have to press down and sort of roll it. When I learned this, I honestly practiced the motion on pens, the steering wheel, my other hand, and various other non-fingerboard places in addition to the fingerboard.