Well, to add tissue you have to have a calorific excess. But the trick is how to partition more of that as muscle than fat. I would guess that the most important variables that control this are:
1) Training 2) hormonal status 3) genetics 4) macronutrient ratio 5) sleep 6) stress and maybe 7) nutrient timing 8) cv fitness
I haven't read anything on this for ages, but my uninformed opinion is that these strategies might help get into category three:
a) Moderate calorific excess. I think the body prefers to add fat compared to muscle, and it probably takes longer to add muscle, so if the calorific excess is too large, you're going to be storing a lot of that excess as fat.
2) Maintain aerobic fitness and work capacity. This alters your hormonal profile in the direction of partitioning as muscle
3) However, not too much GPP and aerobic activity as this may delay recovery and tip you into catabolism.
4) Manage catabolism through training. Sub 45 minute training sessions to maximise anabolic hormones/limit release of corticosteroids.
5) Manage chronic stress as this is related to storage of fat
6) Train for hypertrophy. Provide adequate time under tension, either by lots of sets of low reps or fewer sets of moderate reps
7) Consider macronutrient ratios. Still lots of controversy over necessity of increased protein, but could be worth a moderate increase. Maintain levels of fat.
8) Consider peri-workout nutrition. I don't think this matters as much as total intake over the day, but it would seem to make sense to have nutrients available immediately after the stress.
9) Get adequate sleep. Again, has a big effect on hormnal profiles.
10) Prioritse large muscle groups. Spend most time and energy on those movements that recruit the greatest number of muscle fibres.
That's all I've got.