Dedication, discipline and determination...PE is an exercise in all three (plus patience). Each person's physiology is unique and will respond differently, experiment with any/all of the suggestions already given and pay very close attention to how your body responds. Keep your expectations moderate and be consistent with all your exercises.
I try to boil PE down to very simple rules of physics...to create change, one must use enough force to effect it (but please be safe/reasonable with the amount of force...especially until you know what your limits are). My quickest gains came when I introduced the VacuVin mod to my X40 Bathmate and I was able to (slowly) max-out the pressure that the VV was able to impart. Switching to the MityVac (and a slimmer cylinder) has allowed me to apply more of the vacuum forces along the length axis. I upgraded the hand-pump to a higher-end automotive brake bleeder unit and it can create far more pressure than I can withstand (for now).
Just use your logical brain and figure out what the best practices are for your particular goals and body.