How do you deal with "food pushers"?

If you’ve ever been on a diet or tried eat more healthily, there’s usually someone who’s “pushing” you to eat more food or to eat foods you’ve decided you want to avoid. They could be a family member, loved one, friend or co-worker. Who is the one person who gives the greatest challenge? If you’re able to successfully resist them, what do you do or say to stick to your new eating habits? If they’re successful in pushing the food on you, what’s stopping you from resisting them?