Gaining weight is just as hard for people like us, as losing it is to people that are overweight. It takes a lot of hard work, tons of protein supplement, a heavy diet (i drink slim fast with my meals), and heavy strength training to gain weight. Higher weight and lower reps. Sprint training instead of long distance cardio.
I could use some tips though. It's so hard to gain weight, I wonder why so many people complain about dieting?