L-Arginine. Take it along with L-Ornithine (the second one is stronger, so a supp with it in there will be more expensive). I'd aim for about 3 mg per day of L-Arginine, or you could halve that and throw in .5 mg of L-Ornithine and you'd be good.
Try taking 4 mg and go to the gym. You'll get a pretty gnarly pump, at least I did.
So, I guess, stick with about 3 mg and you should see a difference.