My pecs are always small, so I try to go for definition. I found that 3 or 4 sets of heavy weight on the pec deck helps define the "man-clevage", but as the others said, you can't change the basic shape. Mine are separated by about 1/2", but I got them trained to a point where it's a jagged line, and you can see the individual muscle fibers (due to very low bf). That makes up for lack of mass, I guess.
Charles