Because the penis is a large sack of veins and arteries, good circulation is the best thing one can do for their erections.  Of course a healthy diet and exercise will help with this, but there are a few supplements one can take.  I started taking Gingko Biloba for its ability to improve memory and noticed that I was having stronger erections.  Recently my girlfriend was having trouble getting erections after being on hormones for eight years.  Even with the Ginkgo she was having trouble shooting any cum, so I bought a bottle of Epimedium, also known as horny goat weed.  The stuff works great.  Now she is fully erect before I even penetrate her, and she shoots loads as big as when we first met.  And when I'm on the goat weed my dick grows another inch so I'm more than 10" when I'm normally 9".  Another great way of improving circulatory health is a daily routine of tai chi.  I do 15 mins. a day.