The dictionary defines criticism as: "the expression of disapproval of someone or something based on perceived faults or mistakes ". And although we have an expression called "constructive criticism", that type should be called "constructive feedback". Criticism coming from disapproval is not meant to be constructive. The first thing to always keep in mind is that when people criticize, they are the ones with the problem not you. They have an issue with something and are projecting it on you. The second thing to do is not engage in the content of the communication. Keep the focus on them, not you. You do this by speaking to "intent" and not "content". Intent is the purpose behind a communication. Content is the communication itself.
So if someone makes a rude comment about how you look, the actual comment she makes is the "content'. The reason she is making it is the "intent". Intent can be positive or negative.
The best thing to do is project positive intent. That means you act like she has good intentions even if she doesn't. An example of this would be, "Why thank you for caring about how I look. That is so sweet of you." You have now accomplished a two fold purpose. One, if she is out to get you and you can't be gotten, it messes it up for her. Second you have trapped them into the positive intent. It is unlikely they will say, "No stupid, I'm trying to cut you down. Sheesh what an idiot you are." Instead they will not deny your positive projection and will be forced to go along with the positive direction you set.
What if the criticizer is a parent? With parents, projecting positive intent is absolutely, positively the way to go. If they say you are fat, thank them for caring about your health and well being. If they want to know when you are getting married, appreciate them for caring about your relationship happiness. These kinds of positive projections will melt a parent. Parents in general feel under appreciated by their children. Usually when you positive project on a parent they roll over and start kicking their leg like a dog getting it's stomach rubbed. They will forget all about what they were criticizing you about and bask in your appreciation. Then you can just change the subject.
For more great stuff on parents and all relatives I refer you to Dealing With Relatives (Brinkman & Kirschner)