Honestly, I don't do anything differently myself, I always feel fine...
But I found this article online
http://findarticles.com/p/articles/m...32/ai_81391076
* TWO WEEKS BEFORE: Eat more iron-rich foods to boost your intake of this essential mineral, says Mars Mallari Apherisis, supervisor at the Stanford University Blood Center in Palo Alto, Calif. Women age 19 to 50 should already be getting 18 mg a day; women over 50 and men should be getting 8 mg. A cup of cooked spinach has 6 mg of iron. Also take 100 to 500 mg of vitamin C a day, which helps your body absorb the extra iron, and avoid high-fiber foods, which block iron absorption. Steer clear of foods high in saturated fat; doctors may reject fatty blood.
Nutrition Action ...
More » * FIVE DAYS BEFORE: If you are temporarily taking a synthetic blood thinner like Coumadin (warfarin), ask your doctor if you can skip it until after you've donated. (Long-term users of this type of drug can't donate blood.) Taking supplements, even vitamin E, known to thin blood, is safe.
* THE DAY BEFORE: Drink ten 8-ounce glasses of water; giving blood dehydrates you.
* DURING: To calm yourself, try this: Inhale deeply just before the needle is inserted, and exhale fully as the needle is inserted. Exhaling relaxes your muscles and makes inserting the needle easier. While your blood is being drawn, focus on the rhythm of your breathing. Exhale fully again as the needle is removed.
* IMMEDIATELY AFTER: Focus on your breathing for five minutes before you get up. For the next few hours, eat plenty of nonacidic fresh fruit (like apples or pears), which will agree with your stomach better than acidic fruit, or dried fruit (like apricots) to replace lost blood sugar.
* FOR ONE DAY AFTER: Avoid vigorous exercise.
* FOR THREE DAYS AFTER: Continue your increased iron intake and drink lots of water.