The best way I've found, is to move your wrist in a similar motion as you would opening a doorknob. Instead of just using your thumb and index finger to move the pick, rotate your whole hand ( lay your hand flat, about an inch above a tabletop, now without moving your arm, rotate your wrist so your thumb touches the table. Now rotate so the pinky touches the table. This is close to the motion you need.)
Remember to keep your hand,arm and shoulder relaxed and let the pick do the work.
If your copping a Surf sound, keep the pick close to the bridge (there's more tension there, and the string won't be flopping around as much) Try learning "Miserlou" by Dick Dale ( the cool Surf tune from Pulp Fiction). That'll give your arm a good workout. Remember to keep the sound even, start slow and bulid up speed. Also, I've had better luck with a heavier pick, ( I use either a Jazz III or a Fender JZ extra heavy.)
Hope this helps.