This recommendation is likely to be the most powerful for many lifters, but it may be the hardest to "digest." From my over 30 years of experience with helping guys get bigger and stronger, I'd estimate that the number one reason they fail to gain muscle is their stubborn refusal to gain weight and allow themselves to get a bit soft while in a building phase.
You're either in an anabolic state or a catabolic state, so you'll need to pick one and commit to it, at least for a period of time. Unless you're the rare exception (i.e. an obese 18-year old noobie taking lots of PEDs), any weight gain you experience will be comprised partly of muscle and partly of fat.
Fortunately, you can put the odds in your favor by doing the following:
- Start your building phase when you're under 14% body fat. When you start off leaner, a larger proportion of your gains will be muscle.
- Gain weight slowly. Shoot for between .5 and 1 pound per week, depending on a handful of factors. Older, more experienced lifters should err toward the lower number, while younger, less experienced guys can use the higher number.
- Lift hard. Really hard. You need to create a reason to grow more muscle, after all. So really get after it.
- Keep your calories and protein sufficiently high. As for calories, they should be high enough to ensure a .5 to 1 pound weight gain per week. For protein, shoot for between .8 and 1 gram per pound of total bodyweight per day.