While I'm not a big fan of the writing style and New Age mysticism of the book "The Four Agreements" by Don Miguel Ruiz the core concept of each agreement applies well to body and self-acceptance. Some people can really identify with the book, but for those of us who can't, this is a series of posts distilling it down to the practical.
First Agreement: "Be Impeccable With Your Word" (Don Miguel Ruiz, "The Four Agreements")
The concept here is based in the idea that you are what you do, and that speech is action. Every time you speak negatively of your own appearance or that of others, you internalize that negativity and it becomes part of you.
What we often don't realize is that the words we say in our heads may not have the same impact as what we say aloud, but it does have an effect and there tends to be much more of it. We're taught that it's rude to say negative things, so we say them silently, and to ourselves, and more or less constantly. Most people are not even aware of the sheer volume of negativity in our lives; it has become normal background noise.
Wrapped up in the problem is that we tend to suspect in others the actions we are most ashamed of in ourselves. When you keep a running monologue of criticism of other peoples' bodies, it makes it easy to believe that others are doing the same about our body. From that comes shyness, body shame, and self-criticism around other people.
Being "impeccable with your word" means watching what you say to yourself as well as to others. Each time you find yourself criticizing or reacting negatively to someone's body (your own or someone else's), see if you can cut yourself off. It's okay to simply tell yourself No.
Of course, trying to not think about something is extremely difficult for our brains. There's the old playground trick where you dare someone to not think about pink elephants. The only way it's possible for most people is to try very hard to think about something else. So when you catch yourself in a negative thought, deliberately substitute a positive thought. Find something about the person to compliment, or that is at least neutral. If you can't, at least try to think about something else. How's the weather?
Dialectic Behavioral Therapy is based on this very concept; that the thoughts, body and emotional state are interconnected. Changing one can often affect the others (for good or ill). By policing your negative thoughts, you can elevate mood and improve health. These are the primary benefits of reducing stress in general, and negative body talk can be very stressful.
This is something that no one can wave a magic wand and fix, but with effort you can begin to head off criticism at the pass. Eventually you'll find that instead of having to force yourself to think good things about people, it will happen automatically. The boost in confidence and happiness this brings is well worth the effort.