How Do I?

[xanadu324]

Ok, here's the skinny.
I'm 5'8" and about 102lbs, or so. I'm trying to put on weight, however, no matter what I do, I gain about a pound or so, and then it's gone. I've tried eating more frequently, eating more carbs, working out, nothing seems to work. I know I'm underweight, but how do I rectify that?