...Don't do it on an empty stomach, it's a horrible idea. I don't know if that's something they usually warn you about, but I definitely wasn't told anything, otherwise I would have immediately declined. Allow me to share the story of my affliction:
I went to Publix today to pick up a few essentials. As I head towards the door, a lady stops me, asking if I would give blood. I don't know why it's so impossibly difficult for me to say "no" to them; perhaps I prefer not be perceived as a selfish ass (I'm also quite sure she would have hassled me). So after I get the groceries to my car, I go to the blood bus, do the usual paperwork, temp check, and a blood test (turns out I'm A+, which I had forgotten completely).
So I give my blood, same deal as my last 2 times. My whole arm started to ache near the end of it, though it's probably because my veins were a bit more constricted today. Afterwards, they give me some CapriSun and a couple bags of chips, and I am on my way.
I was going to head home right after, but I then decided I would go enjoy the juice and chips over by my favorite ice cream place (a potentially live-saving decision on my part). I spot the table and chair outside the shop and head over to take a seat, and the moment I sit down, I instantly get dizzy. I figure it was due to a quick change in body position; I usually get dizzy from standing up real fast. It didn't fade though; it got worse. It got harder to breathe, and I was getting nauseous. I started drinking the juice, thinking it'd do me some good.
No good was coming of it.
About halfway through the pouch, my vision darkened, like when you've been outside on a sunny day and as soon as you come inside, everything looks dark. As my vision started getting blotted with dark spots, my hearing became very strange, like I was underwater. It was around this point that I may have passed out.
After waking from my unconscious state, I proceeded to vomit the previously ingested juice onto the cobbled walkway. I guess it wasn't very agreeable. A few minutes afterward, the lady who owns the ice cream place came outside to help me out. I came inside and she handed me some water, a couple napkins, and bag. She even gave me a free scoop of ice cream. After a few hours of recuperating, I was ready to head home.
So, avoid giving blood on an empty stomach, unless you like passing out and yacking.
(Also, potato chips and ice cream taste pretty good :9)