I started at about 132lbs and 5'10'' tall. I've put a fair amount of weight but when I look back I wish I had a journal so I could really see what was working for me. Now i'm not sure and I still blunder my way from one routine to the next wondering of this will be the on that really works for me.
If you started out a skinny little s**t like me but have had great success what would you account it to?
Disclaimer: The information below is in the context of my first bulk in college, and doesn't reflect my current activities as my body type has completely fu**ing changed since then and now I have to eat perfectly to make muscle gains/fat losses.
I started ~140 at 6'2". My first bulk in college took two years and I gained 70 pounds while staying under 9% bodyfat. My metabolism was out of control, and I had almost no size gains at all (strength gains came as a result of stronger mind-muscle connections) until i did a few things.
Eat. I was taking in on average 5,000+ calories a day, and still sitting at the same weight. I didn't know about maintenance calories or calculators that could show me about how much it would take to gain, I just stuffed as much chicken, tuna, salmon, lean beef, turkey, rice, brocolli, and sweet potatoes as I could into my belly. All day. I ate every 2 hours, regardless of my hunger level. Nowadays, I listen much more closely to my hunger levels, but i still get protein in me every 2-2.5 hours.
Lift heavy, intensely, perfectly, and on a progression. I stayed in the 10-12 target rep range, but I would add a bit of weight to where i could only do 8 or so. I would push that until I was able to rep my target range, and increase the weight again. You are too small to have an ego about the weight you are lifting, so don't think a second about starting small in order to meet and push your rep goals.
The BEST thing I ever did, was to focus on form at first, and am still reaping the benefits today as I bulk for only the second time in my life. I read books, watched videos, asked the trainers at my gym, everything I could to make sure my form was perfect. That baseline is the most important part of actual lifting you can do. Once you have your form down, you have to increase the intensity. Intensity and load are the key to gains, so read up in the exercise science and training forums here about drop sets, super sets, forced and partial reps, negative reps, etc. There are tons of ways to increase your set intensity, just learn when, how, and how often.
Don't supplement. I didn't touch a thing other than whey isolates and amino acids (only in my second year did I learn about amino acids). When you are first starting out, your body will most likely adapt extremely quickly. You only stunt your future potential by supplementing during your newbie gains. No NO boosters, no fat burners, nothing until you are months and months into your routine and have experienced gains on your own. When you do finally add supplements (creatines, nutrient partitioners, vasodilators, endurance, fatburners etc) to your diet, add them slowly one at a time so that you can test their affects on you in a more controlled manner. You need to know which ones your body responds to or not, which ones affect you adversely, and which you just didn't like. Add a supplement to your diet only when you determine a need for it. Don't just throw a bunch of herbs/chemicals into your body because someone tells you it'll make you huge or such. Do it to enhance your diet, which is the key part of overcoming the hurdle of being a hardgainer.
Be glad you are a hardgainer.
It probably means you don't have to worry about gaining fat weight for a while, and it forces you to do things right in order to progress.
I lost 15 pounds of that muscle over a 6 year period, and after realizing how much of a fatass I was becoming started working out again about 3 months ago. But I learned a lot by being a hardgainer. I couldnt just lift heavy things and get bigger like my dad (strange, our genetics were completely opposite), and had to research to do it right. I'm glad for that.