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.
Okay Spunky, go to the mirror and take a peek at your backside. Careful, chances are the view back there is a mess. Chances are you ain't got no meat back there. That's right, no muscle, or very little.