An article in Science by Nicola Lacetera, Mario Macis, and Robert Slonim summarizes their research on economic incentives and blood donation (abstract; PDF). Contrary to previous studies, the researchers found that various incentives, from gift cards to a day off, increased blood donation:
Overall, 18 of the 19 distinct incentive items offered in observational and field experimental studies increased blood donations, and the effects were larger for items of higher monetary value; only one reward offer, a free cholesterol test, had no effect. When data were available (for 15 of the items), no effect on blood safety was detected. Finally, although temporary rewards might affect long-term motivations, no post intervention effects on donations were found, including any negative effects deriving from potential motivation loss.