Rallen,

http://www.alphahorse.com/pick-hoof.html does a wonderful job of explaining it! Instead of clicking (that to me means move over or away from me) I always used the word LEG. With my mare, she would lift it without me ever touching her as soon as I said LEG. The vet and farrier loved that!

When I have worked with other horses that knew how to pick up their feet, and were just being stubborn, I would lean into them as I ask for the foot. I also did this when I was teaching the little ones to give me their foot. This gets them off balance a little and they are more likely to hand you their leg. But be warned, some of the big guys will lean back when you are holding their foot. I swear, you can almost hear them giggle as they put all their weight into that one foot. Stinkers!