Pomegranate JuiceResearch at the University of Southern California and at University of California Los Angeles has shown that Pomegranate Juice can promote a better quality erection.
It's Obviously The AntioxidantsAs quoted in Best Life:
You should drink eight ounces of pomegranate juice each day for firmer, stronger, erections... The juice's high anti-oxidant content keeps arteries free of plaque, allowing for increased blood flow.
In addition to drinking more pomegranate juice you can also look into horney goat weed as it accomplishes much of the same blood flow benefits the the antioxidants do.
On a fairly different train of thought you might also find interest in reading about some of the dangers of antioxidants as not all antioxidants are as great as one might think.