Your chest is made up of two muscles, the pectoralis major and the pectoralis minor. The pectoralis major, located at the top of the chest, is the larger of the two and is responsible for depression of the arms as well as rotating the arms forward. The pectoralis minor is beneath the the pectoralis major and is responsible for moving the shoulders forward and down. These muscles must be developed and you must know how to move your body to isolate a muscle contraction before you can flex your pecs like a pro bodybuilder.
Some exercises target different parts of the chest and will help you get a better flex. To develop the top of your chest, do a combination of cross-over push-ups, close grip push-ups, Smith machine bench presses, reverse grip bench presses, dumbbell pullovers and lower pulley cable crossovers. To build muscle bulk so your pecs will pop when you flex them, choose a weight that allows you to complete five to eight reps, and rest for three to five minutes between sets.
Mid and Inner Chest
In order for your pecs to pop when you flex them, you must define the crevice in between your pecs by doing inner and middle chest exercises. Include bench presses, reverse grip bench presses, dumbbell presses, cross-body cable presses and cable crossovers to accentuate the separation between the two sides of your chest. Do three sets of six to 10 reps for each exercise and rest for two minutes between sets.
A developed lower chest keeps your pecs from looking saggy and gives them more of a tight and upright look when flexed. For stronger lower pecs that help your chest look more defined and lifted when flexed, do a combination of decline bench presses, Smith machine decline bench presses, flyes, push-ups, parallel bar dips and high to low cable crossovers. Choose a weight that allows you complete five to eight reps, with three to five minutes of rest between sets, to develop the strength and muscular power that will allow you to flex your pecs.
Once you have developed a well-rounded chest you'll be able to actually flex it and look good doing so. To flex the pecs, you must trigger a muscle contraction in the same way as when lifting weights for this muscle group. Stand in front of the mirror so you can get visual feedback for what you are doing. Pump your arms in toward the center of your chest, bring your shoulders in toward the center of your chest and rotate your shoulders down to get blood circulating to your pecs and initiate a muscle contraction. Identify how it feels when your pecs contract so you can isolate that muscle group when flexing. Keep practicing until you are able to get your pecs to pop more and you can hold your flex longer.