Dave you wouldn't put the diodes from the 7915 output to ground but from
the output to your load in series......being a negative polarity output
you'd put the cathode towards the 7915 (and any additional diodes as well).

The trick to bumping up the voltage of a 3 terminal fixed regulator is
to insert a diode between the ground pin and ground (keep in mind to
insulate the tab if you do this!). The regulated voltage will now be
pushed up by the junction voltage of the diode or diodes (Diodes placed
as such in a 7912 would have their cathode face the ground pin.....anode
to ground).

clear as mud?