Triggerman saidThe post about diamond push-ups was pretty good. I would also throw in wide push-ups to hit outer chest. The problem with flyes and bench is that the tension on the pecs stops at the top. I would do cable flyes and make sure to cross over hands at the end. The great thing about cables is that the tension stays throughout. Most guys do cable cross-overs wrong by standing in front of the pulleys. That kills the tension at the end. Make sure the pulleys are well in front of you. Make sure the machine is in front of you not at your sides. Take two steps back. You will still have tension at the end when the inner pecs, come into it. The other trick is to pull your shoulders together in front of you at the end as if you are trying to form a U shape from shoulder to shoulder. That builds the inner pec which sits under the outer pec. This builds mass UNDER the outer chest and pushes the top layer, the outer chest up like a rock under a blanket.
The two work differently. The outer chest pulls the arms across the chest. Typical fly movement. The inner chest pulls the shoulder forward and together. You can work inner chest without arms just by pulling you shoulders forward and together. So, it is two layers and they do two different things hence two different muscles. Do both. Exercises that pull the arm forward and inward and also do some that really only move the shoulders forward. Like hunching you back. Most only do outer chest which is great but doing inner chest builds a small mountain under the mountain and pushes it up and out.
Too much info? Hope not. Hope it helps.
Read what he wrote...thrice...and memorize it.
And to add my $0.02, be sure to let your body rest for a couple days after intense training.
If you don't have access to a gym for that, use a ball (volleyball, soccer ball, basketball, etc). Hold the ball directly in front of your chest with elbows bent at a 90 degree angle, and attempt to crush the ball. That will have a similar effect.