Gaining Weight?

Well, unlike most people I am actually underweight (no I do not have, nor have I ever had an eating disorder). I'm about 100 pounds and 5'7" and I cannot gain weight, I have the metabolism of a rodent...The only thing that ever worked for me was pigging out on potato chips and little debbie snacks (lol) but that's reallllllly unhealthy. Anyone have some tips for me?