You can't work or develop single areas of a muscle group just place different parts at different mechanical advantages (or disadvantages more appropriately). Like most guys said you need to increase your chest as a whole for systemic rather than a specific part, focus mainly on the mechanical function of the pec, which is namely to bring the humerus across the chest, plain and simple that's the function of the chest period. So with that being said and with in mind the fan structure of the fibres across the chest you really should focus on some form of flat (minor incline) dumbbell work, some form of decline press and a cable flye movement, would'nt go with dumbbell flyes as the chest is low on tension the majority of the movement due to our good friend gravity only working in one direction. Just my old man opinion.