So what is this powerful tool?
It's a workout logbook and a pen. And that is not joke.
These things may not be the most exciting things in the world but the truth of the matter is that a workout logbook and a pen is the basic foundation for which your entire training program should be based around.
The reason for this is simple and that is that the ultimate foundation for building muscle size and strength is progression, In other words, if you want to consistently build your muscle mass to greater and greater levels, then every single time you enter the gym you need to present your muscles with a new growth stimulus that they haven't experienced before.
Every time you go to the gym you are breaking down your muscle tissue and then forcing your body to adapt to that specific level of stress that you presented it with. But once the muscles have fully adapted to that specific level of stress, they are not going to grow any bigger or any stronger until you increase the workload or increase the stress and force them to adapt even further.
Just to illustrate, if you've been bench pressing 180 pounds for 6 repetitions from two months ago until now, your muscles are most likely not benefiting from that anymore (i.e., they are not stimulated by it and are not growing from it anymore). What you need to do is to increase the weights and the number of reps incrementally every week or every two weeks.
And this is what makes your workout logbook and pen very important. Each time you go to the gym, make it a habit to put on record everything that you did--the exercises you performed, how much weight you used, the number of reps and sets you executed. Doing this will allow you to determine exactly what you have to do to give your muscles that added stimulus the next time you go to the gym. Using a workout logbook to record your training and progress takes out much (if not all) of the guess work out of your training.