How can I stop my puppy nipping/biting?

Hi there, hope you can help.
I have a 12 week old black labrador bitch. She is pretty good, but when she gets playful, the nipping and mouthing starts. It's really quite painful, and my hands are now covered in welts and marks. I've tried the yelping and then stopping the game to no avail - the yelping just gets her even more excited! I think she's getting a little stir-crazy - her 2nd set of jabs are tomorrow, and then we've got a week before we can start her on walks. Any suggestions?

Thanks.

You need to teach bite inhibition This article by Ian Dunbar explains how to go about it: http://www.jersey.net/~mountaindog/berner1/bitestop.htm