I've always been incredibly scrawny ever since junior high.
Currently I am 6'0" and 115 lbs.
While I DO work out every other day, I was skinny even before starting that regiment. I attribute it to the fact that during junior high, I pretty much cut almost all the unnecessary sugars and oils from my diet. By unnecessary I don't mean I stopped eating things that taste good, I simply never touched a soda, and most fruit juices again. I also laid off the dairy unless explicitly called for in a recipe.
When my diet lost all the sugar and gained more proteins, fibers, and complex carbohydrates, I think it contributed immensely to the maintenance of my figure.
I've also always drunken a lot of water, pretty much my entire life. These days it's pretty much the only thing I DRINK, but even as a kid I drank a lot of water between the glasses of milk and juice. Maybe that has something to do with it.
But now that I'm older, I attribute my consistent low weight more to the exercise I'm getting, as I'm sure my metabolism has started to slow down or will within the next few years, so diet will become more of a supplementary factor.
But honestly, cutting soda and concentrated fruit juices alone can have a tremendous impact after less than a year. Candy too.