Time under tension. If you do any lifting and follow any of the numerous YouTube videos, you've heard of time under tension.
What does it mean?
Basically it means how long your muscles are under a strain while you're performing a set. Instead of grabbing dumbbells and popping out 8 - 10 reps in 20 seconds or so, you slow it down. Take one second to raise the dumbbells and then slowly lower it for 3 seconds. A 20 second 10 rep exercise becomes 40 seconds.
This works to tear down the muscles to a greater degree which, in turn, causes the muscles to grow faster.
It works. Another way to increase the time under tension is to do drop sets. Let's say you curl using 50 lb dumbbells. You manage to get to 8 reps and cannot do another. Drop the weight to 30 or 35 lbs and squeeze out as many more as you can.
It's tough. It works. Give it a try next time at the gym.