I don't know if this will help, but you could try drinking a WHO ORS electrolyte powder packet in room-temperature water just before donating blood. Or you could drink half before, and half after. Drink it slowly, and practice first at home.

Or if the fainting is psychological, then work your way up to psychological dominance via multiple blood tests rather than donations.

I used to have this problem until I was given the tip to tense various muscles throughout my body during the process. Do not listen to the nurses who tell you to relax!