There are two approaches you can take to this very common problem.

Either will work. One is more work than the other. And some people say one is more gentle and humane than the other.

You will need to choose one technique and consistently apply it.

(1) Use your knee

This is the one that even non-dog owners know about.

The dog comes up to you, quivering with excitement. He rears up to place his paws on your stomach or chest. You turn sideways just before he is able to put his paws on you, and raise your knee so he meets a hard, pointed-but-blunt object. After hanging on that for a second or two he drops to the ground. You win.