When I'm asked to teach just one quick technique that can be used in lots of situations and is easy to learn, I teach the Boring Baroque Response (BBR). Suppose you have to deal with someone who is forever coming at you with hostile attacks like "WHY can't you EVER do your share of the WORK around here??" and "WHY do you eat SO MUCH JUNK food??" and "WHY don't you stop DRESSING like a NERD??"
What your attacker wants is an interaction that goes roughly like this:
This gives your attacker a chance to run you through a long list of complaints about the way you eat, and to demonstrate his or her power to really get you going. Even if you come out of this thinking that you have "won the argument," you've lost -- because the attack worked, and the attacker got what he or she wanted. People like your attacker are like little kids who'd rather be punished than ignored: If the only way they can get your full attention is to get your negative attention, they'll settle for that.
Instead of falling for this tactic, use a Boring Baroque Response. Your attacker has come at you with "WHY do you eat SO MUCH JUNK food??" And here's what you say, while you stare not at the attacker but off into space, as if you were thinking deep thoughts.
A response like this delivers the following message: "I notice that you're here to pick a fight. Do that if you like, but it's not going to be much fun for you, because I won't play that game." Listening to a BBR is excruciatingly boring. The most usual result is that by the time you've gotten to the part about your aunt's dog the attacker is already saying, "Oh, never MIND!" and leaving in a hurry -- while making a mental note that you're no fun as a victim and shouldn't be chosen for that role in future.
When the attack comes in the form of a statement instead of a question, as in "ALL YOU DO is stuff your face with JUNK food!!", just begin with "You know, hearing you say that reminds me of something that happened to me when I was just a little kid...." and so on. If you need a hifalutin version, say it reminds you of "an article I read only the other day in the New York Times. No, wait a minute.... It couldn't have been the New York Times. It must have been the Washington Post , because that's the one that comes on Thursday and Eileen always gets it before anyone else and....." . The BBR is also the best way to deal with none-of-their-business questions and comments from strangers. Like, "Oh, what a cute baby! It looks Chinese! [Or Spanish. Or whatever. The nosy stranger's point is that whatever the baby looks like, it doesn't look like it shares your ethnic heritage.] Where did you GET it?" Just remember one thing: You have to do the BBR straight. If you sound sarcastic or patronizing or hostile, it becomes a counterattack and it won't work.