In my page on Arterial Stiffness, I discussed some of the causes of hardening arteries. The reasons are not dissimilar to what happens to a rubber hose left outside to age in the sun and the elements. In the case of your arteries, including those that supply blood to the penis, plaque buildup and damage to collagen and elastin slowly do their work over the decades, leaving you with an inflexible, non-expandable blood supply pipeline.
So is there any hope, any relief for us guys who want to mend and heal their bodies naturally? Well, of course, you cannot completely undo the damage from years and years of abuse from a modern, industrialized lifestye, but it is suprising how much elasticity you can build back into your cardiovascular system.
Below we outline some of the key research in this area:
1) Testosterone. One study after another has shown that low testosterone levels significantly accelerate arterial stiffness.  In fact, researchers have noted that Prostate Cancer patients that go on anti-androgens have significantly stiffer arteries. Check your testosterone levels, especially if you are experiencing any of the classic Symptoms.
2) Aerobic Exercise (Cardio). Aerobic exercise helps with almost every known cardiovascular risk factor, so why not with hardened arteries as well? Indeed, that is the case  and that is one of the reasons that some experts recommend always doing aerobics after weight lifting. The arterial stiffness resulting from the high blood pressure spikes of heavy weight training can be significantly relieved by a little cardio right after the fact, something I cover in my link on Weight Lifting and Your Arteries. The effects of aerobic exercise is so strong in this regard that some researchers have noted that it can undo much of the hardening that occurs naturally with aging.
3) Body Fat. Body fat does many nasty things to us guys, including increasing estrogen and oxidative damage, but one of the worst is accelerating hardening of the arteries.  Yet another reason to drop that gut and replace it with some nice, tight abs that will get the Little Woman's attention, eh? There is also evidence that weight loss actually improves arterial stiffness. 
4) Low Salt Diet. As I discuss in my link on Arterial Stiffness, salt has been implicated as one of the primary artery-hardeners. It will probably be no surprise to find that there is evidence that a low salt diet can do the opposite. 
5) Garlic. One study on seniors found that garlic significantly slowed down hardening of the arteries.  It will also lower blood pressure too.
6) Fish Oil. Almost everyone knows how good fish oil is for the heart due to its inflammation and triglyceride-lowering capabilities. Not as well known, though, is the fact that at least one study showed it increasing circulatory flexibility. 
7) Viagra. As you may know, we are not a big fan of long term use of the either Viagra or Cialis, due to their high number of side effects. However, one thing they apparently one thing we can say is that they seem to increase arterial elasticity. 
8) Dietary Nitrates? Nitrates, as in the medications, improve arterial elasticity. There is no reason to think that dietary nitrates will not do that same to one degree or another. In my page on Erectile Dysfunction and Hypertension, I mention that beet root juice does a nice job of lowering blood pressure. How does a simple glass of juice lower blood pressure so significantly? You guessed it - a high nitrate content.
9) Low Fat Diet and Pomegranate Juice? One of the underlying causes of hardening of the arteries is arterial plaque buildup. Both a Low Fat Diet and Pomegranate Juice are likely to help, although I know of no study proving this.
1) Nephrol Dial Transplant, 2011, "Low serum testosterone, arterial stiffness and mortality in male haemodialysis patients"
2) J Am Soc Nephrol, 2004, 15:2713-2718, "Impact of an Exercise Program on Arterial Stiffness and Insulin Resistance in Hemodialysis Patients"
3) Circulation, 1993, 88:1456-1462, "Effects of age and aerobic capacity on arterial stiffness in healthy adults"
4) Archives of Internal Medicine, 2005, 165(8):875-882, "The metabolic syndrome, cardiopulmonary fitness, and subcutaneous trunk fat as independent determinants of arterial stiffness : The Amsterdam growth and health longitudinal study"
5) J Hypertens, 1999, 17:1831-1835, "The effects of weight loss with or without exercise training on large artery compliance in healthy obese men"
6) Arteriosclerosis, 1986, 6:166-169, "Improved arterial distensibility in normotensive subjects on a low salt diet"
7) Circulation, 1997 Oct 21, 96(8):2649-55, "Protective effect of chronic garlic intake on elastic properties of aorta in the elderly"
8) Arterioscler Thromb, 1994, 14:1425-1429, "Fish oil impr ves arterial compliance in non-insulin-dependent diabetes mellitus"
9) Vascular Medicine, 2003, 8:243, "Effect of sildenafil on arterial stiffness and wave reflection"
6) Am J Physiol Regul Integr Comp Physiol, 2010 Oct, 299(4):R1121-31, "Acute and chronic effects of dietary nitrate supplementation on blood pressure and the physiological responses to moderate-intensity and incremental exercise"