Once you are on the bench & the dumbbells are in the air. Start by rotating your hands ever so slightly. This will create more emphasis on the pecs.
Slowly lower the weights but do not over stretch & take the dumbbells beyond the height of your chest.
Keep the pace 3-1-2 . 3 seconds on the eccentric, 1 second on the static & 2 seconds on concentric . This way you will create more tension on the muscle fibres & lower the risk of injury too.