I'll try to be as politically correct as possible, but hold your forearm on a table or desk, but have you wrist and hand off the surface...allowing it to move freely.
Simply repeat moving your wrist up and down with a closed fist. Repeat up and down at a reasonable pace until you're going in roughly the same angle up and down...then increase your speed...this should work the muscle and make your wrist recognize the motion.
Also, try holding you arm horizontally in mid-air and move your wrist up and down as fast as possible....
Remember to use only your WRIST MUSCLES
Then try it with a pick on your guitar...start off slow
Hope this helps.