10. Bad stuff. Please don't say "I want to gain weight" if you're still drinking and smoking. When I was smoking, I literally could not gain weight, even doing 800 calories over my maintenance diet. Just don't do it. Cut out the bad stuff, drugs, etc out of your life if growing is your goal.
11. Keep a Journal. If you do not keep a journal, how do we progress in weights? Do we do what feels good, or do we just leave the bar where it is? Every month, try to go for a new personal best - even if its adding those 2.5 plates on to your barbell bench. Thats 5 pounds added a month, or 60 pounds added a year. Its not embarassing to add a 2.5 plate. Its embarassing being the same size a year from now, when every one around you is getting bigger.
12. Mass Gainer exercises. There's a few exercises that are "mass gainers", proven to stimulate the most muscle fibers of said target muscle group. Most of these are compound exercises (except barbell curls). Always START your workout with them, as you will be fresh, and able to do the most weight. I'll pick the most common workout - back/bi's. If this is you, Do one back mass builder, then the bicep mass builder. Do three or four sets of 10 reps. Then do the other set. Then start your routine. Mass builders are called such, not only because of the muscle fibers stimulated, but the sheer amount of weight you can do. No tricep workout will have you doing more weight than dips or close grip bench. Most won't even have you doing half. Start with these on the appropriate days, and make it happen. Also, so many people say "I don't want big legs, just a big upper body, so I don't work out legs." As you can see from the following exercises, legs in paticular, that its a big mistake. Also, squats are proven to add about 15-20% to your UPPER body mass, by virtue of the free flowing testosterone that squats release.
back - pull ups
back - deadlifts (increases free flowing testosterone by 15%)
biceps - barbell curls (cheating)
biceps - chin ups (weighted if needed)
calves - donkey calf raises
chest - barbell bench (increases free flowing testosterone by 8%)
chest - dips (body forward, legs back)
hamstrings - straight legged deadlifts (ask Kai Greene) - forces your hamstrings to act as the main stabilizer muscles.
legs - squats (raises free flowing testosterone by 20%)
legs - the sled (a.k.a. leg press)
shoulders - military press
shoulders - upright rows
triceps - dips (weighted if needed, body upright, legs as straight as possible)
triceps - close grip bench
overall - overhead squat (best growth hormone booster)
overall - clean and press
13. stress. This is an Edit in reference to something I mentioned in a related thread. Stress will seriously mess with you. It can affect how often you eat, how much you eat, and completely unmotivate you in the gym. If something is bothering you, figure out how to fix it. Put a positive spin on your life - with me its "Well, my friend with type 1 diabetes doesn't complain much, what do I have to stress about?"
If you're doing all these things already, and not gaining, try DC training - I'm not gonna mess with Dante.
Special thanks to everyone on this forum - a lot of points are mentioned here that I personally would not have added (drinking alcohol), because I simply don't drink, and therefore think nobody else does. You guys helped inform me, and I'm compiling all the data into one place. If I missed anything, please let me know - will edit it accordingly. I can't stress this enough: YOU GUYS ARE THE BEST, AND MAKE THIS FORUM WHAT IT IS - THE TOOLS TO SUCCEED.