It is best to tap behind your leg, but it on all honesty, it doesnt matter as long as you don't crack him on top of head. It's about getting his attention. You have leg on LEG ON and crack! whip says, "hey!, listen up" He knows what you are asking, so a quick slap of the reins on the neck is ok too. It's a wake up gesture.
Some horses need you to carry a whip all the time. Sad to say.
One thing that helps it to set up some sort of game ala barrel racing. Use cones or other items and try to "race" from one place to another. Having a clear destiantion, esepecially when the horse knows what it is, as he will after several repeittions, means moving out with a purpose. When horses chase cattle, they move with a purpose, and they really MOVE!