I give blood every 2 months (as often as allowed). Can't do the double-red thing, have a reaction to the anticoagulent used in that, so just give whole blood. Did it yesterday, as a matter of fact.

One thing to be added to this: Selling your blood, while populare in certain low-income crowds, isn't the same thing. In the US, purchased blood can't be used medically, so its main use is in cosmetics (it's an ingredient in the binding compounds used in lipstick - and knowing that makes lipstick more than a little icky). To save lives, the blood has to be donated through specific organizations. Make sure, before you donate, that it'll go to medical use, not be sold to cosmetics companies.