I just threw out my back/pulled a lower back muscle maxing out my squat last week. Suffice to say that I'll add one thing to this discussion:
ALWAYS WEAR A WEIGHT BELT WHEN SQUATTING OR DEAD LIFTING
Yes even *you* who thinks you don't need one. That is all.
Edit: Ok, I can't resist adding another.
I've always found it easiest to add muscle mass by doing mostly compound movements in 3-5 sets of 4-8, always to failure. The key is keeping the lifts compound, and the weight high enough that you are never able to do more than 10-12. If you can do more than 10, you up the weight.
Namely just a few exercises: Bench, Upright Rows, Dips, Military Press, Cleans, Clean & Jerks, Pulldowns/Pullups, Squats, Deadlifts, and Lunges.
I really think this is one of the main things that new lifters ignore, these compound movements. When performed correctly, they target multiple muscle groups and increase both strength and size gains very quickly.
And lots and lots of protein. Chicken, Greek Yogurt, Black Beans, Chicken, Mixed Nuts, Eggs Whites, Chicken. I was talking to a high school kid who was trying to get bigger recently. He was doing all the right exercises. I asked him how much he was eating. He replies, "Enough". Turns out on lifting days, including powder, he was eating around 80-100 grams of protein. Probably closer to 50 on non-lifting days. You really are what you eat. And muscle is protein.