Like G.D. said, make sure your thighs are parallel to the floor or sloping downwards when you raise your heel up. When you're not playing the bass drum, the heel should be resting on the ground, and it should be raised only when you're preparing for a stroke. Now, there are basically two main variations to the heel up technique -- leg motion and ankle motion. The leg motion is used when brute force is needed, and the ankle motion is used for lighter dynamics, finesse and speed.
When playing the bass drum, it's a good idea to let the beater rebound off the head. This will improve your control over the drum's sound (if you can play open strokes, it's easy to "bury" the beater when required, but the opposite isn't necessarily true...), make you faster and more relaxed, and lessen the strain on your foot and leg. Start working on this by playing consecutive strokes at a slow pace. Just dribble the pedal with your foot, keeping the beater in constant motion. Experiment with both the leg motion and the ankle motion. When you start to get a feel for the rebound, play five consecutive strokes, landing your heel on the ground right after the fifth one -- the motion should be fluent, as if the pedal board's rebound led your foot to rest. Once the "resting motion" becomes familiar, play fewer consecutive notes, until you are just down to one stroke.
Heel up shouldn't require one to hold his heel up constantly (let alone having the beater against the batter head), but only when necessary.
