One of the best ways to boost nitric oxide actually has to do with your tongue. That's right - bacteria in your tongue can convert the nitrates in food to nitrites and then that is converted to the precious nitric oxide that you need in your gut. Dr. Nathan Bryan - and you can read My Interview With Dr. Bryan - has noted that this is the most effective and efficient way for the typical man over 40 to boost his nitric oxide and yet has been largely ignored by the medical community.
Of course, the issue is this: how do you consume those nitrates? There are foods that are high in nitrates, and I discuss some of them in My book The Peak Erectile Strength Diet. But there is another way as well: food extracts. One of the foods very high in nitrates is beetroot juice and one study after another of late has verified its superpowers:
WARNING: Mouthwash can sabotage the above nitrate conversion process that occurs on the tongue. See my link on Mouthwash and Your Sex Life for more information. It is like that toothpaste does the same thing, although to a lesser extent. Read this link on The Dangers of Toothpaste to understand more.
1. Blood Pressure. One recent (2010) study found that 250 ml of beetroot juice, particularly in men, decreased blood pressure in dose-dependent manner.  This means that the more beetroot juice consumed, the more that blood pressure lowered.
DOSAGE: If you are going to just drink beetroot juice itself, the 250 ml used in the above study is equivalent to about 8 oz. or 1 cup (for those in non-metric countries). Of course, the 500 ml study is 2 cups, which would allow you to take two doses morning and evening. If your going to use powder or extract, you will need to go by the manufacturer's specifications. Beetroot juice is not terribly sweet either or high in fructose, so there is little concern there.
2. cGMP (Cyclic guanosine monophosphate). The above study found that 250 ml of beetroot juice increased cGMP. This is the Holy Grail for erectile dysfunction: the whole purpose of increasing nitric oxide is to boost one's cGMP, since it is what relaxes the arteries. Furthermore, PDE5 is the enzyme that breaks down cGMP and the PDE5 inhibitors (Viagra, Cialis and Levitra) all work by slowing down the activity of that enzyme. Again, the goal for stronger erections is more cGMP.
3. Erections and Erectile Dysfunction. Can beetroot juice help a man who has erectile dysfunction? There is no study yet verifying this, but it is very likely. Anything that increases blood flow and cGMP and nitric oxide significantly is going to improve erectile strength.
4. Brain Blood Flow. Okay, so we know beetroot juice increases blood flow throughout the body. But what about the brain? Would it benefit as well? Scientists found that beetroot juice's benefits crossed right through the tricky blood-brain barrier and increased blood flow in key areas of the brain relating to executive function. This study was in seniors, but there is little doubt that blood flow will be increased for many decades downward as well.
What is executive function? It is the idea that there is a region of the brain that manages the others. Executive function involves many creative and multitasking processes, for example. Poor executive function leads to poor career and social performance and interaction for many people. Beet root juice may be a critical help for people as they age. And many studies have shown that the early stages of Alzheimer's show a loss of executive function. 
5. Exercise Endurance. Scientist aren't quite sure how it works, but 500 ml (2 cups) of beetroot juice each day boosted how long they could ride their bike by 16%.  This may not seem like much, but the authors noted that it was the biggest known way to boost oxygen including training! This could translate to a 2% decrease in run times for athletes, for example.
Many more studies will come out on beetroot juice and, undoubtedly, find many more benefits. Right now about 1-2 new studies are coming out each year and, so far, they all seem favorable. For a great all-around summary on the power of beets, see My Review of Beet the Odds coauthored by Dr. Nathan Bryan.
DANGERS? Processed meats have been on the hot seat for decades now and things have not gotten any better since researchers first started looking at it. One prominent theory is that the nitrates in processed meats are sometimes eaten with amine-containing food and create nitrosamines, a highly suspected carcinogen. Study after study has shown, for example, processed meat consumption tied to stomach cancer , colon cancer , childhood leukemia (in the case of hot dogs)  and possibly Type II diabetes. 
And so the theory goes that it is the nitrates and nitrities in processed meats and red meats that cause the cancers. However, there is a big problem with this theory: there is no expert or study that I know of that thinks that spinach or beets, both of which are high in nitrates, increases cancer risk in any way. Nor have I ever seen anyone cautioning the consumption of natural, plant-based nitrates in whole foods or juices.
The explanations for what is occurring centers around five cancer-causing compounds that are in or result from animal products but not plants. Below we list these five compounds, which create a formidable list indeed: 
1. High fat diets and the ensuing insulin resistance
2. Heterocyclic amines (HCA's)
3. Carcinogenic N-nitroso compounds in meat
4. Heme iron in red meat (which increases cell proliferation)
5. polycyclic aromatic hydrocarbons (PAHs)
So we'll have to wait for a few more years for a definitive answer.
1) Hypertension, 2010, 56: 274-281, "Inorganic Nitrate Supplementation Lowers Blood Pressure in Humans"
2) Nitric Oxide, 2011, 24(1):34-42, "Acute effect of a high nitrate diet on brain perfusion in older adults"
3) Neuropsychology, Jul 1995, 9(3):313-320, "Executive function deficits in mild Alzheimer's disease"
4) Ann Oncol, 2004, 15(2):346-349, "Processed meat and the risk of selected digestive tract and laryngeal neoplasms in Switzerland"
5) JAMA, 2005, 293(2):172-182, "Meat Consumption and Risk of Colorectal Cancer"
6) CANC6) CANCER CAUSES AND CONTROL, 1994, 5(2):195-202, "Processed meats and risk of childhood leukemia"
7) Nutrition and Cancer, 2008, 60(2), "Processed Meat and Colorectal Cancer: A Review of Epidemiologic and Experimental Evidence"
8) Carcinogenesis (2007) 28 (6): 1210-1216, "Processed meat intake, CYP2A6 activity and risk of colorectal adenoma"
10) Journal of Applied Physiology, Mar 1 2011, 110(3):591-600, "Dietary nitrate supplementation reduces the O2 cost of walking and running: a placebo-controlled study"
11) Diabetologia, 2003, 46:1465-1473, "Processed meat intake and incidence of Type 2 diabetes in younger and middle-aged women"