I rarely get called (ab neg), but when I do, I just do a 1/2 pint. I don't have the body mass to do a full pint.... I also have extremely low blood pressure. That, coupled with ADD... well, not a good mix. It takes forever just to get a dang cup of blood out of me. I can't stand it, it's worse than getting my hair colored.
Shameless plug... I do it for for for my mother's memory as opposed to altruism, she founded the blood bank of alaska. She drilled into me how precious blood is.