I'm a skinny guy (183cm, 66kg), so I decided to go on a GOMAD diet.
Basically it stands for: Gallon Of Milk A Day
For all the non-US people it's about 3.8 liters of milk. That's a fucking lot.
A gallon (or about ~4 liters) contains approx. 2400 Calories. Combined with meals and protein shakes, this can make your daily energy intake at about 6000 Calories, which is humongous.
Of course, you have to workout while maintaining this diet. You can't just consume all this food and sit around all day. Squats are suggested, as well as any other multi-joint excersize like deadlifts or bench presses.