"The Perfect Child"

So how do you raise a child so that they don't think that they are perfect? I am not sure how. My husband and I try to give "creative criticism" to the kids, but it always seems to backfire. They take it as though we have told them they don't know anything. For example, the tweenager is a great runner, so we told her she would be even better if she increase her food intake and ate better nutrition wise....well that went over like a lead balloon. "What, you don't think I can beat my friend? I am not a good runner?" I can't....I'm not...stomp the feet, slam the door....
How can you talk to a child and let them know there are better ways of doing things without them thinking you are really criticizing them?
HELP!