Yep, take a look at this assertiveness primer. Please read the whole thing, then revisit the paragraphs on Fogging and Negative Assertion.
They are both ways of defusing somebody who's trying to get a rise out of you by criticising. I think of them as "Yes Dear" techniques 
Take a look at this post, where BreakFree tried it for the first time last night :)
Some other good bits are a slow, deep breath and count to ten before responding (yes, make him wait.) The breath puts you back in charge of your voice, and the counting lets you look at the situation rationally instead of doing a knee-jerk.
Counsellors teach you to imagine an invisible shield, which protects you from all the barbs and arrows of verbal attack. It's best done with a trained counsellor, but you can do it yourself. Do it when you feel calm and safe. Breathe nice and slow, then imagine your shield (mine's made of blue light, yours can be whatever you choose.) Your shield expands and contracts wherever you are - in the car, it sits snugly round you; in a big hall there can be several yards of space, but it always covers and protects you completely. If you like, you may furnish and decorate it to your liking. The important thing is that it's all yours, and it's safe.
You can flick up your shield at will. As soon as anyone has a go at you, get your shield round you and just watch as those nasty words some flying at you, only to bounce away without touching you. I like to think some of them bounce right back at the speaker! It's nice to see them being hurt by their own bullshit 
Another counsellor's trick is the thumb-in-palm memory jogger. To do this, think of a time when you felt perfectly at ease, calm, happy, safe and secure. Really recall where you were, what you could hear, see and smell, and how you felt. Hold it. Press the thumb of one hand firmly into the palm of the other - count to at least ten. This fixes your safe feeling in that gesture - any time from now, if you feel unsafe, press the same thumb into the same palm to regain your feeling of being at ease.
Oops, didn't mean this to be so long! Do have a read of the assertiveness page :)