Do not lock your elbows out at the top of the movement. This takes the strain off your chest muscles.
Use a slow an controlled movement. It should take about twice as long to lower the weight as it does to raise it.
Do not set the bench angle too high as this will involve too much of the front shoulders and take some of the emphasis off the chest.
Instructions
Set an incline bench to an angle between 30 and 45 degrees.
Grasp two dumbbells and sit down on the bench, resting the dumbbells on your thighs.
Using your legs to help push the dumbbells up, lay back on the bench and push the dumbbells straight up above your chest, keeping your palms facing in. Bend your elbows slightly. This is the starting position for the exercise.
Slowly lower the dumbbells down until they're almost at chest level. Allow your chest muscles to stretch out.
Pause, then contract your chest to move the dumbbells back to the starting position.
Do not lock your elbows out, then repeat the movement.