Get your upper back off the top of the bench. This allows the scapula to move with more range of motion, allowing the pecs to get into a more lengthened state.
Keep your palms forward and bring the dumbbells back toward your ears, not down to the side of the body. This also puts the pecs into a deeper stretch.
Use only a slight bend in the elbows. Most people get way too much fixed elbow flexion with flyes, turning into more of a half press/half-flye type of movement.
Some call it the ultimate triceps movement, others call it, "that exercise that hurts the top of my head and makes me see birdies." Either way, you ought to be doing Partial Overhead Presses.