I was skinny pretty much my whole life and decided a couple of years ago to get in shape. I did a few rounds of P90X, but wanted to get bigger.
I've finally hit on a routine that seems to help me grow. I started doing Stronglifts 5x5 about 9 weeks ago and have gained 6 pounds over that time! 5x5 focuses on the three main lifts (Squats, Deadlifts and Bench Press) with a couple of supplementary lifts. 3 workouts a week, and you do squats every workout. 5 sets and 5 reps per set, and you add 5 lbs. to the bar each time you do a particular exercise. That means adding 15 lbs./week for squats.
I currently weigh 153 lbs. and today I squated 225 lbs. for the first time! In 2 weeks, I'll be squating 260 lbs.!