This is from my first book (The Black Book of Training Secrets)
Getting lean and muscular through posing
I vaguely remember being a frail 14 years old boy almost lost in a gym, hearing a big bodybuilder shouting, "Hey guys, today is mirror training". For these guys, who were competitive bodybuilders, "mirror training" or posing, was important because it could influence how they would do in their show.
One thing I noticed is that when they were practicing their posing they would seem to increase their muscle density and their degree of leanness at a much faster rate than usual. This intrigued me to no end. Now I understand that posing practice actually involves a lot of muscle control and maximal static and quasi-static muscle contractions. While not sufficient to stimulate hypertrophy gains, this type of training can increase energy expenditure as well as improve neural activation of the muscles (thus creating more tonus at rest).
Don't get me wrong, I don't recommend devoting a training day to posing in the gym (freaks!). However, when your goal is to get very lean and dense, including a weekly posing session lasting 30-45 minutes (preferably at home when you're alone!) can really make a difference in your appearance. I would recommend holding each of the mandatory bodybuilding poses (there's 7 of them, 8 if you count the "most muscular") for 60 seconds and repeating the cycle 2 or 3 times. Concentrate on fully contracting the muscles at all times, the contraction must be maximal.
This advice may sound vain or narcissist, but it's actually grounded in science! Muscle control really is improved and so is resting muscle tonus. In turns out that this will make you look denser, more muscular, and will actually help you with your lifting performance. Now, who's up for some "mirror training"
As an addition what I find is that flexing a muscle improves your capacity to recruit that muscle when you do it often enough. The better you are at maximally flexing a muscle, the better you'll be at using that muscle in a lifting exercise and thus the more effective your lifting exercises will become.
Recruiting and contracting a muscle is a motor skill. The more often you practice it the better you become at it.
The material from the book was written with bodybuilders in mind, but you can simply focus on flexing a muscle as hard as possible without actually posing.