I once saw this blog about how naturally skinny people stay that way. I don't know how true the tips are, because I'm naturally skinny myself, so obviously I didn't read it. Though I'm kind of curious to see if the tips list things I do. *shrug* I'll look for it for you if it's still on the interwebz.
EDIT:
OK, I'm almost positive this is the one:
http://skinnyobservations.blogspot.com/
Again, I can't vouch for how true the tips/techniques in the guide are, but I read that description about the "evil skinny people", and was almost insulted. Exactly *how* many people want to slap me? lol
Other than that, count calories? gah, sorry for not being much help. But I get strangers asking me for diet tips all the time, and I don't know the difference between a calorie and celery.