To become a pec bench presser, you have to set up your pectoral girdle. Take the bar from the rack and roll your shoulders under your body. Feel as though you’re pushing your rear delts down toward your waist, and keep pushing them down hard throughout the set. Never let up. At the same time, arch your sternum (chest bone) up. As the weight goes up, push through with your pecs, not your delts. Keep your shoulders down and back.
In addition, move the bar in an arc, not straight up and down. The arc allows you to contract your pecs hard at the top. I suggest that you practice with light weights until you get the motion down pat.
Use the same delts-pushing-down, chest-pushing-up position on flyes so you keep your delts out of the movement as much as you can and your pecs as the primary movers.
‘Greg Zulak, IRONMAN’s Ultimate Bodybuilding Encyclopedia