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.
Christian serves up 3 innovative methods for taking your gains to new heights - one for biceps, one for arms in general, and a Bizarro World method that involves doing the opposite of everything you now do.
I'm always experimenting in the gym. Hell, I sometimes think if I didn't spend so much time trying new ways to train that I'd be a lot further ahead, physique-wise.