Only advice I can give is eat a good meal about 2 hours before and drink plenty of fluids before and after. The donation team at my sessions give you a pint of water before you donate. And insist that you drink it all. (this has reduced the fainting rate. Which is very low anyway, except for the college students who weigh about 8 stone, don’t eat then turn up and faint!)
Yes, your iron levels do drop slightly after donating, but with a healthy diet will be back to normal within a week or 2. I’ve done a few different trials over the years, double red cell donation, monitoring iron levels after donation, reduced times between donations.
53 donations so far, so they’ve definitely taken more than an arm full.