Lay down flat on the bench. Pull your legs as far back close to your head as possible, this will allow for a good arche. Now pull your posterior delts onto the bench, arche your back and stabalize with your lats. Now come down to your lower chest, approximately your sternum, breathe deep, and exhale and explode off your chest. Make sure your elbows are IN, not out. very important.
Remember, a big bench doesnt mean a big chest. In order for a big becnch, you need a good back, strong triceps, and strong shoulders, and of course, strong chest.