Many people not only want to get stronger but are looking to add size to their chest. Think about it, adding more size to your chest effects the appearance of your midsection, and makes your waist look trimmer as well. Of course there are many ways to accomplish this but I have found that the low-incline fly has been overlooked and works great at adding depth and width to the chest. Many people do the flat dumbbell fly, which hits the central/lower/outer area of the chest. If you adjust the angle of the bench to a slight incline you hit the muscle at a new angle which is a great stimuli for growth. Don't increase the angle too much or you will be using more of shoulders. That's why I recommend adjusting the bench so that it is at 20-30 degree angle, located somewhere between the flat bench and a normal incline bench. By raising the bench too high you will be using supporting muscles which you don't want to do. See if you can find an adjustable bench at the gym since most incline benches are positioned at a 45 degree angle.
Know that you have the angle of the bench down, let's talk about the motion of the lift. Angle your motion a little upward over the upper end of your chest, as you raise the dumbells. As you bring the weights together you can turn your thumbs slightly towards each other. When performing this lift make sure to lift very heavy and slow for size. When doing this exercise it is easy to tear a muscle if you aren't careful. By lifting slow you also take away any momentum that would have helped you cheat. Although I recommend you lift heavy, start off with lighter weights to get the form down. Once you can do ten reps with ease start using heavier weights. You'll be surprised how much mass you can add to your upper, outer chest simply by making some adjustments, to an exercise you're probably already doing.