Overall chest mass (and most often times the desired "cleavage" effect) is simply attained by pure mass in the sternal head of the pectoralis major. The sternal head is by far the largest part of the chest muscles (1 of 3 and about 80% of the total) and size is best acquired through maximum muscle fiber stimulation. Flat bench presses (dumbells being superior to barbells) stimulate more muscle fiber than incline bench presses or dumbell flyes (cables and machines being better here). As far as form, the shoulder blades should be pulled together and the bodies natural range of motion should be followed. In addition, the upper arm does not need to go much further below the parallel plane before anterior delts take over so the ROM (for chest only) is not nearly as large as most people insist upon.
As for Incline vs. Flat: The clavicular head is most often times thought to be stimulated the most during an incline bench press. However, most physiologist will tell you that the clavicular head gets the most stimulation from pullups, flyes and lateral/front raises. Anterior delts tend to take the brunt of most incline bench presses and actually have a tendency to put the shoulder joint at a less than ideal angle for safeties sake. However, follow proper form, keep a spotter on hands at all times and don't allow your shoulders to move at all and you can still
activate a large portion of the clavicular head - you just won't get a massive chest from it.
Inner pec vs. outer pec stimulation: On
