I used to weigh 167lbs at 6'3. I ate 5-6 meals a day and lifted 4 times a week.
Now I weigh around 217. I eat 5 meals a day and lift 4 times a week.
The two big things were:
1) Cutting down on my extra activity -- if you're skinny there's a good chance you're running, playing sports all the time, etc. To crack 205 all I did was lift...any extra activity was cutting into my ability to recover from my workouts.
2) Counting my calories: I couldn't gain weight for the life of me. I got really frustrated and told myself I'd do whatever it took. I started eating 5 meals a day at 600 calories. After my weight stopped moving up I ate 5 meals x 700 calories. Then, a few weeks later it was 5 x 800 calories. A few weeks later it was 2 x 900 and 3 x 800. Finally I was eating 5 x 900 and I got to a point that I was satisfied with. I found a low-fat diet worked best for me (didn't gain much fat at all until I got sick of eating 900 calories of low-fat food 5 times a day)
My workouts are pretty much the same as when I was smaller. One thing that helped for a while was capping the sets in a workout. I would do 8-12 sets max and then leave the gym. Now I can do maybe 20-30 and feel fine, but initially I was overtraining on 15.