My buddy is on a program... it caters to people who are 'hard gainers' and have a tall, lean, slim body type.
Basically it calls for downing a 'shake' in addition to their meals... but this shake consists of stuff that should put lean body weight on... you really want to avoid merely putting any kind of mass to gain.
Its what the above people have stated already... increase your caloric intake with 'good' fats + fiber, protein, carbs. So he eats what he usually eats + a shake... which is about 1000 calories. It has good fats from 1 cup nuts/seeds (almonds, walnuts, cashews), 1-2 cups spinach, 30-40gms of protein, and then fruit (banana, strawberries, blue berries).... blend the crap outta it with almond milk or water (or ice). Its about 3 cups worth each meal.
Hes been on this program for about a month or so and has put on 10lbs of lean muscle.
(Hes a 'hard gainer' ... 6 ft, 170lbs... now hes at 180)