I am always looking for ways to naturally boost my nitric oxide levels and one of the best ways is probably one the cheapest also: Vitamin C. Vitamin C is the only supplement that I megadose and the reason is that I pretty well buy into what is called "Pauling Theory", which points out that we are one of the very few mammals that does not manufacture its own Vitamin C. To date, there are huge benefits to Vitamin C and, in my opinion, very few downsides under rather special circumstances.
But let's go to every guy's favorite subjects: boosting blood flow, increasing nitric oxide, lowering blood pressure and improving erectile strength. Vitamin C does all of these. It can also lower cortisol and decrease inflammation, something I discuss in my link Why Take Vitamin C? On this page, though, I want to focus on Vitamin C's NO-boosting powers and here are just a few of the studies that show, directly or indirectly, that Vitamin C can raise nitric oxide:
1. Dose Dependent NO Increases. Dose dependency is what every researcher hopes to find, because it shows that the more you give of something, the greater the effect, which shows a more clear relationship. And this is exactly the case with Vitamin C and nitric oxide. One set of researchers stated it like this: "ascorbic acid [vitamin C] has been shown to stimulate endothelial nitric oxide (NO) synthesis in a time- and concentration-dependent fashion." 
This same study even found how Vitamin C works to raise nitric oxide levels: it protects (from oxidation) a cofactor eNOS called tetrahydrobiopterin. A cofactor is "a substance that acts with another substance to bring about certain effects - especially a coenzyme." So, assuming you have the baseline nitric oxide in the first place, which you can likely boost with either citrulline or nitrates (in food), then Vitamin C should raise nitric oxide levels for you.
2. Reduction of Nitrite to Nitric Oxide. One thing that I talk about in my Interview with Dr. Nathan Bryan is how nitrate in food is converted to nitrite by bacteria in the mouth and later to nitric oxide in the gut. Vitamin C is a similar miracle worker and converts nitrites to nitric oxide. One set of study authors noted that ""Vitamin C is an effective scavenger of nitrite, reducing it to nitric oxide and preventing nitrosamine formation in vitro and in vivo." Thus the protection of tetrahydrobiopterin is not the only way that Vitamin C can increase a man's baseline nitric oxide levels. (There is a study out there that says, in the presence of significant fat in the stomach, nitrosamine formation is actually increased. See my caution below.)
3. Restoration of Endothelial Function in Heart Disease Patients. Heart disease patients with arteriosclerosis are plagued with "endothelial dysfunction", which means that the lining of their arteries cannot pump out the sorely needed nitric oxide for lowering blood pressure/increasing blood flow/getting an erection. (The lining of the arteries is called the endothelium.) However, Vitamin C therapy has been found to help restore endothelial dysfunction in these cases, which is remarkable considering the severity of the situation. And what is more remarkable is that this occurred at the relatively small dosage of 500 mg/day.  If Vitamin C can help arteries and improve blood flow in heart disease patients, it can likely do it for you too!
4. Increased Citrulline Levels. A lot of men are taking Citrulline to improve their erectile dysfunction and you can read about it in the page on The Benefits of Citrulline. What is interesting is that an in vitro study showed the Vitamin C can actually increase citrulline levels. 
DOSAGE and FORM: I am currently taking 3 doses of 500 mg split throughout the day. I take the "Ester-C" form, which is calcium ascorbate with a small amount of some other herbs and extracts thrown in to increase absorption. Ester-C is a trademark or brand name However, you can also buy calcium ascorbate from other supplement manufacturers, but it will not have the exact name Ester-C. Of course, you can also take Vitamin C as ascorbic acid, but I have tried this and it gives me heartburn. This is why most people take the buffered forms.
NITRIC OXIDE TESTING: What is the best way to test your baseline nitric oxide levels? That is actually a fairly involved question, but there are new nitric oxide test strips available from Neogenis that can indirectly do the trick for you in the privacy of your own home. What they actually measure is nitrite on the tongue and you can read the technical details in this link on At Home Nitric Oxide Evaluation Test Strips to find out more. Keep in mind that Dr. Bryan, the inventor of these test strips, recommends that baseline readings be taken first thing in the morning before any activity, eating, brushing of the teeth or mouthwash, etc.
CAUTION: Vitamin C should probably be taken on an empty stomach by most people, because it can increase iron absorption. Most men do not need increased hermatocrit/hemoglobin and/or iron stores (ferritin), due to red meat consumption and other factors. Elevated iron levels have been implicated in heart disease and various brain conditions. (Iron literally "rusts" the body.) There is also a study out there that shows, if you have over 10% fat in your stomach, Vitamin C can actually increase nitrosamine formation. (This is a controversial finding however.)
1) The Journal of Biological Chemistry, Jan 5 2001, 276:40-47, "L-Ascorbic Acid Potentiates Endothelial Nitric Oxide Synthesis via a Chemical Stabilization of Tetrahydrobiopterin"
2) Circulation, 1999, 99:3234-3240, "Long-Term Ascorbic Acid Administration Reverses Endothelial Vasomotor Dysfunction in Patients With Coronary Artery Disease"
3) Free Radic Biol Med, 2000 May 1, 28(9):1421-9, "How does ascorbic acid prevent endothelial dysfunction?"
4) Gut, 1989, 30:436-442, "Vitamin C in the human stomach: relation to gastric pH, gastroduodenal disease, and possible sources"
5) Circulation, 1999, 99:3234-3240, "Long-Term Ascorbic Acid Administration Reverses Endothelial Vasomotor Dysfunction in Patients With Coronary Artery Disease"
6) The Journal of Biological Chemistry, Mar 19 1999, 274:8254-8260, "L-Ascorbic Acid Potentiates Nitric Oxide Synthesis in Endothelial Cells"