Almost every guy thinks he knows what works, but, just as you've found with pleasing your girlfriend, there is usually a better option. By taking three exercises you've already been doing and giving them a slight tweak, you can fully unlock the muscle-building potential in your pecs.
Exercise 1: The Dip
Secret Tip: Move your hands out wider so you get a deeper stretch in your pecs when you lower your body. V-shaped dip bars are an ideal way to do this, rather than the parallel ones you're used to using.
The Payoff: A wider grip position helps you recruit even more muscle fibers.
Exercise 2: The Dumbbell Bench Press
Secret Tip: Start with your palms facing each other in the bottom position and your elbows tucked to your sides. As you press, rotate your wrists so that your palms face forward in the top position.
The Payoff: The twisting motion activates more pec fibers overall.
Exercise 3: The Dumbbell Fly
Secret Tip: Try the move using a towel on the floor. Fold the towel in half and place one palm on each end of it and get into pushup position with your knees on the floor. Move your arms apart as in a normal fly and lower your chest toward the floor.
The Payoff: It's hard, working your pecs, back, and core.