While deflecting criticism may seem daunting, it can be done with practice. However, you never want to come off as defensive, both for your own sake and for the perception others will have of you. The ability to ward off the judgments of others has more to do with your self-esteem than anything someone can throw at you.
Deflect gingerly, but do not act defensively. You can and should defend yourself in battle, in football, or when baseball-sized hail is pummeling your person. Do not defend yourself when someone starts spouting off critical remarks, or you will look even worse than you did before you opened your mouth.
Practice deflecting criticism in the mirror or in the car on your way to work. Get a good friend to throw mean comments your way: "You look like you've gained a ton of weight since you had the baby!" "I doubt you'll be able to pass French this semester." "Your stylist must be on vacation." While some criticism in life is deserved and key to our growth, the very word "criticism" means that others or another at least will be passing judgment on us. One could argue that criticism is never actually necessary because even when a teacher, parent or boss gives us a review or critiques our work or behavior, they too are fallible.
Know the difference between criticism and encouragement. A friend may have known you 20 years and it pains her to see that you've gained 40 lbs. since Ricky the Snake broke your heart. If she tells you gently, "Come walk with me. It will be good for you," do not take that personally. Keep in mind that it is hard for friends to tell us what we need to hear; they aren't perfect. But if they are friends, they do have our best interest at heart, and once you shed even 10 lbs. you will be thanking your friend for piping up.
Summon your wit. Nothing combats a detractor more than uncommon wit.
Put everything in perspective. If you are pro Obama and your friend is pro McCain, remember that nothing she said today about your knee-jerk liberalism will be important in six months when you have gone on that girlfriends weekend in Yosemite together. We are put on this earth to love one another, yes, but also to express contrary views. And as corny as it may sound, being able to voice dissenting views is still one good thing about being an American.