I say this because I ended up going into the ER the other day with chest pain, palpitations, and shortness of breath only to find out my hemoglobin levels were well below where they should be, much less where they actually were before giving blood the week before. Anemia fucking sucks.
In particular, if you give blood regularly, keep your iron intake up, take a supplement if you need to, and don't be afraid to stretch out the distance between donations more than just the bare minimum. A 6+ month wait between donations is better than having you get all anemic.