Giving blood is the option I hear about most.
It seems that it can be difficult sometimes though, I have heard of people getting refused for strange reasons as well as the limit on the number of times you can do it.
This may sound crazy to some, but has anyone done their own phlebotomy? I know crazy cyclists that do it and then add it back in before their competition.
I have done a lot of 'at home medical treatments' such as removing moles/skin tags, stapling bad cuts with a staple gun, burning bleeding wounds, draining infected wounds, and drawing blood to measure my own hematocrit using a centrifuge.
It doesn't sound that difficult to do, especially with one of those butterfly needles you can get online.
I cut my ass cheek open so bad the other day, it severed a blood vessel and bled for 3 days straight. I had a lot to do and couldn't go to the hospital (didn't want to pay either), so I stapled it shut and then stapled some paper towels and duct tape over it. Worked like a charm, and no infection (I did sterilize all the equipment used).
It had bled so bad, I bet I lost enough blood to consider it a phlebotomy, lol. Too bad I sold my centrifuge, it would have been cool to compare the before and after hematocrit levels.