For building the chest, incline the adjustable bench only to about 30 degrees. That's normally the lowest setting possible on commercial gyms.

A lower incline is even better. If your bench doesn't get much lower, simply place a weight plate under one end of a flat bench. This is much better for the shoulders and allows the "upper" pecs to do most of the work.

Bring the dumbbells close together at the top without touching them. Never clang them together like a dork.

Related:  The Very Best Way to Build Your Chest

Related:  16 Nasty Exercises You've Never Tried