I find that masturbating after PE works well if you wait 30+ minutes to allow your penis to recover a little, I get my hardest erections if I do this and I try to expand my shaft beyond maximum size by erect kegelling from time to time.
Edging (holding back your cum several times) really helps too, most of the time, 10-15 minutes of this without ejaculating helps keep me horny all week. When you do allow yourself to cum after edging a few times, you get the most amazing orgasm.
I recently found that only ejaculating once or twice per week really helps keep my flaccid penis larger most of the time and helps with my EQ (erection quality).