Eat lots of the right foods in the right ratios. 40% carbs, 30% protein and 30% fat would be a good start. There's debate as to whether this is more effective or not for adding muscle but many small meals is the easiest way to increase your intake. Way too many hardgainers eat high protein diets with practically no carbs and wonder why they can't put on weight. Fat is essential (no pun intended) - you need plenty of EFA's and should also load up on peanut butter and bags of mixed nuts for between meal snacks. If you want to gain a few pounds quickly increase your water intake to 8 pints per day and supplement with creatine. Oh yeah, don't forget to lift heavy 3 or 4 days per week. 5 x5's is good but you might also want to cycle it with some hypertrophy training depending on how your body reacts.