I have donated bone marrow three times. All were during my undergrad years, and the motivation was supporting medical research but also that it paid well (from the perspective of an undergrad).

The donation process was exactly as described in http://www.marrow.org/Registry_Members/Donation/Donation_FAQ... but I was given only local anesthesia. The marrow was withdrawn from one of the two protrusions of my pelvis about 2-3 inches away from my spine (basically the very top of my butt). The process is certainly a strange feeling, but it's tame--not donating a kidney or anything.

It's a good opportunity to practice deep breathing and relaxation while focusing on the wonders of the human body.