Puppies: How to stop the biting

[Dactyl]

My parents have had Timmy, who is a miniature schnauzer, for just over a month. IIRC he's about 20 weeks old now. He's generally quite well behaved , however, he's still quite bitey. In particular; hands, toes and legs. My parents have been employing the "ouch" and ignore method but it doesn't appear to be making a difference and my Mum is starting to get frustrated that she doesn't know how to discourage it properly.

So I beseech thee oh wise snopesters, what are your techniques for stopping puppies from biting? And by what age should they have stopped?