I'm both naturally skinny and a vegan. Healthiest part of being a (non-cheating) vegan? You eventually assume that everything has milk in it until proven innocent. So you check the ingredients on everything for awhile.. and then you soon get lazy and start buying stuff with 10 ingredients, tops. Plus, you actually know what the hell those ten ingredients are. Why vegan? It's easier for me to just do something vs. half-ass it. (I went vegan over a glass of milk, of which I drank like water)
Basically this has gotten me off of: all candy, pop, most/all processed foods, etc, and I feel awesome. I now love the vegetable aisle, because everything there has basically one ingredient in it.
For all of you who religiously insist on bacon, um... just find a diet that has an ingredient that's in lots of stuff, and make up some ethical reason for following it, or something, and then stick to it religiously. It works.
Oh, yeah. Exercise/no stress is definitely the most important. As in don't eat super unhealthy shit, and as long as you're relaxed and exercise regularly, you'll do better than anyone who has a super healthy diet, but is super stressed and/or doesn't exercise.