Lemons: They're not just for fighting off scurvy! The vitamin C you find in all that is citrus-y, along with peppers, potatoes and strawberries, increases blood flow.
That's important because pressurized blood is what gives you an erection -- the arteries that go to that oh-so-crucial area open up, while the veins going out of it narrow. (You can learn more about the process in How Viagra Works.) Your blood is now where it needs to be, and since it has nowhere to go, your penis has to accommodate it somehow. Vitamin C helps this process happen.
An added benefit for older men, who are more prone to them to than younger guys: Vitamin C can help you get rid of a urinary tract infection (UTI) [source: NKUDIC].
Be careful not to take too much, though (never more than 2,000 milligrams -- stick with 90) -- we can all agree that nothing ruins the mood quite like diarrhea.