This is a question I can answer with complete confidence and authority. I am a phlebotomist for a major blood bank.
Here are my tips:
1. Get enough sleep.
2. Eat normal healthy meals at your regular time. Don't donate just before mealtime (when you are hungry) but instead an hour or two after your meal.
3. Lots of fluids. After you have your mini-medical, drink a 16 oz bottle of water before they actually draw the blood (take a potty break if you need to--they will wait, I assure you)
4. Wear light layers and prior to donating, peel off enough outer layers so you are just on the cool side of comfortable.
5. If you tend to get dizzy, ask that your head be kept low, or your feet elevated. Also, if they have ice packs or chemical cold packs, have them place one under your neck during the donation
6. Don't look if it bothers you--the only person who needs to look is the one who is inserting the needle.
7. If you have unusual pain, or feel sick, hot or dizzy say something immediately!
8. When you are finished and ready to get up, do it slowly--sit on the side of the bed for a minute or so with your feet dangling.
9. Stick with cold or cool liquids until your next meal. Don't rush to get hot coffee.
Thanks for being a blood donor!