Go to the bar after you give blood! :)
That's a very good thing your doing, though. They say the Country is low on blood supply.
I don't do it for a number of reasons - the first being me and needles do not get along. There has to be a major compelling reason for me to allow anyone to stick a needle in me.
All kidding aside, I generally do not qualify to give blood because of my weight, rather my lack thereof.
My final reason - I'm a smoker and there have been too many reports of smokers being denied blood transfusions and organ donations because they smoke. Additionally there have been numerous reports of the blood and or organs of smokers being rejected just because the donor was a smoker.