Whats the proper way to hold a cat

It seems like every time I go to hold a cat, it leaps out of my arms and scratches me in the side with its back claws. How can I hold a cat so that it won't leap away or scratch me?

I hold all of my cats with their rear paws in one hand (they are standing on my hand) and their front paws on my shoulder. This lets them feel like they are totally in control and can jump away whenever they want without needing to scratch or bite. They usually purr up a storm and nuzzle my face because they like the way I respect them. Sometimes I carry them in what my wife calls "the football hold," one arm underneath the whole cat supporting him while he leans against my stomach/chest -- again, in this "football hold" the cat is actually standing on the level part of my arm.

The fact of the matter is, even the tamest of cats don't usually like to be carried around much. Cats are independant critters and have definite ideas about who is supposed to be in charge (them). If we had a steering wheels or reins they might like us carrying them about more, but sadly we don't. What most cats want is for you to sit down and then they can sit on your lap and be petted and sritched. My cats especially like for me to sit down in the recliner chair, put the foot support out, and then they lie on my extended legs like they were in a hammock. They'll stay there for hours while I read a book or watch TV.

=== EDIT ===

Ah, since you have updated the question to state that you are training for veterinary work, it is a completely different question! In this case you are dealing with stressed-out cats who don't know you and you don't have time to establish a bond with them.