Well, you should probably just shell out a few bucks and get your doctor to refer you to a nutritionist of some sort - they could tell you how to gain weight in a healthy manner.

If it were me, I'd try to add a few hundred healthy calories to the meals i normally eat - aka, I'd add almonds or walnuts to breakfast cereals or yogurt - things like that. If you normally eat toast slap some peanut butter on it...

Another thing would be to add one last meal in before the end of the day a couple of hours before bedtime - nothing big, but - again, healthy and calorie-packed but not too heavy on the stomach such as nuts and the like.

Lift heavy weights at low reps to build muscle and strength for your upper body - mine was getting a bit small, too, but adding a weight-lifting routine has helped a lot.