The Mediterranean Diet is one of the most heart healthy diets on the planet and studies have shown that it both helps existing and prevents future erectile dysfunction.  This diet emphasizes olive oil, nuts, whole grains, fruits and vegetables, moderate red wine and dairy coupled with limited meat consumption. This combination is just what the doctor ordered: the well-known Lyon Diet Heart Study showed that after an average follow-up time of about four years, patients following the Mediterranean-style diet had a 50 70 percent lower risk of recurrent heart disease. That's great news for your heart and great news for your sex life! This was re-verified by Italian researchers who tested the erectile-improving capacities of the Mediterranean Diet on men with Metabolic Syndrome and found that it significantly improved their erectile capacity. 
The Mediterranean Diet also has also been shown to dramatically reduce the risk of dying from cancer and heart disease.  For details, see my link on How to Improve Your Mortality and How to Protect Yourself Against Cancer.
NOTE: The poster children for the Mediterranean Diet are the Ikarians, residents of a Greek Island where one in three residents live to be 90!  (This is in market contrast with America, with its expensive health care system, where about 1 in 9 do.) One of their secrets is a Mediterranean Diet, with lots of grains, fruits, vegetables, fish and olive oil. Oh, and AARP reports that most Ikarians over 90 are probably sexually active - that means 70+ years of partying!
So why does the Mediterranean Diet help so much with your health and sex life? There are many reasons, but here's just a few to hopefully motivate those of you who have grown up on the typical western diet:
1. Nuts. Most nuts counter the ill effects of saturated fat. Saturated fat tends to cause endothelial inflammation and stiffen the arteries and blood vessels - which of course is bad for penile blood flow - and nuts counteract this effect. Walnuts, in particular, have been shown to do this, but all nuts such as walnuts, almonds, pecans, pistachios, hazel nuts, and peanuts produce favorable results.  And let's don't forget that nuts also stimulate the release of an amino acid called arginine, one of the building blocks of nitric oxide. And nitric oxide, as any veteran of this site knows, is the building block of erections.
2. Olive Oil. Olive Oil has many excellent properties. In the same study above on walnuts, researchers found that olive oil had the same vessel-protecting effects as walnuts, just to a lesser degree. Olive oil is also inversely related with cognitive decline in a study or two. In other words, the more Olive Oil, the better your brain functions with advancing age.  But, that said, this is the one part of Mediterranean Diet that I urge caution on. I document recent concern How Olive Oil Is Bad For the Heart.
3. Saturated Fat. The Mediterranean Diet demphasizes Saturated Fat - a little Saturated Fat is good by the way - and Saturated Fat has been shown to decrease blood flow and stiffen blood vessels. That's not what you want downstairs. You want nice, relaxed, comfortable, happy blood vessels that allow blood into the chambers of your penis - well, that is if you want an erection. We also know that studies have shown that blood pressure is correlated to erectile dysfunction. The DASH Diet - just Google for more information - has been shown to strongly lower blood pressure and the DASH Diet heavily deemphasizes Saturated Fat (even more than the Mediterranean Diet).
4. Fruits and Vegetables. Fruits and vegetables have also been shown to reduce coronary heart disease by 20% in one study and remember that the heart and the penis are one. 
5. Red Wine. Moderate drinking, 1-2 drinks/day, is associated with elevated good cholesterol (HDL) and decreased heart disease. Remember that HDL is a potent stimulator of Nitric Oxide. It also is associated with greatly reduced risk of Alzheimer's Disease in the well-known longitudinal study called PAQUID.  For moderate drinkers, 1-2 glasses/day, the risk of dementia/Alzheimer's was reduced by about half.
On top of these benefits to both your erectile and cardiac strength, a Mediterranean Diet has also passed the most rigorous of all tests: mortality studies. The results of these studies have shown that the Mediterranean Diet improves overall mortality, which means that it is good for both your heart and multiple cancers, etc.  In the following link I also show that the Mediterranean Diet is pro-testosterone, pro-heart and anti-cancer. (By the way, the Mediterranean Diet is also good for the brain. See this link for more details.)
Another reason that the Mediterranean Diet probably help with erectile dysfunction is that a recent study  found to improve Metabolic Syndrome . Metabolic Syndrome is a set of conditions (high blood pressure, insulin resistance, poor lipids, etc.) that plague Western societies. There are several interesting things about this study. First of all, the participants were seniors, all of whom were at risk for heart disease including about two thirds who already had developed Metabolic Syndrome . The study found that the Metabolic Syndrome was actually reversed for 14% of the participants. (NOTE: Nuts were also a part of the most succesful cohort in this study.)
And that's not all: the Mediterranean Diet has also been shown to prevent Alzheimer's Disease and dementia.  Furthermore, the greater the adherence to the Mediterranean Diet, the less the risk of Alzheimer's. Those with the greatest adherence to the Mediterranean Diet had a 40% less risk than those with the least. For more information on the Mediterranean Diet, see this link. A previous study found a similar link.  So it's not just the heart and penis that desperately need you to follow the Mediterranean Diet: it's the brain itself.
For those of you who are Mediterranean Diet aficionados, you'll be glad to know that a recent meta-analysis found that most studies showed a decreased risk for becoming obese for those who followed a Mediterranean Diet.  Again, the Mediterranean Diet is relatively high fat (~30%), making it easier to put on weight. However, it does convey a low glycemic load, which may contribute to its ability to keep off weight gain. And one could also speculate that perhaps because it is fulfilling and satisfying, it leaves its practioners eating somewhat less.
What is the Mediterranean Diet? Well, one of the mortality studies defined it as "characterised by a high intake of vegetables, legumes, fruits, and cereals (in the past largely unrefined); a moderate to high intake of fish; a low intake of saturated lipids but high intake of unsaturated lipids, particularly olive oil; a low to moderate intake of dairy products, mostly cheese and yogurt; a low intake of meat; and a modest intake of ethanol, mostly as wine." The other mortality studies have a similar definition.
CAUTIONS: Be careful about too much fish: you can get too much mercury and other heavy metals rather easily. Fish Oil, preserved with mixed tocopherols, is probably the safest alternative. Alcohol deserves mostly praise, but a certain amount of caution as well.
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