What are the best tips to prevent yourself from feeling dizzy or passing out after a blood test?

Position: If you know that you are prone to passing out after a procedure like a blood draw, stick, exam or such, then request to lie down to have it done. Elevate your legs higher than the level of your heart and drink lots of liquids prior to the testing. Try to thing of other things and and take slow, easy and deep breaths but not more than 10-12 per minute. Some are just have to deal with it.

Lie flat: Lie flat before you are tested. Stay flat after the procedure until you are ready to get up. Let the venipuncture person know. Often the person who has the best physical conditioning (i.e., triathletes, varsity athletes) has the most vagal tone and gets woozy before anyone else, so it's no reason for embarrassment -- i'd be more worried about anyone who doesn't dislike having blood drawn.

Prepare: Drink plenty of water and avoid dehydrating drinks like coffee. Ask the phlebotomist to explain the procedure well. If you need to be fasting, keep a candy in your pocket to suck on later. Rise slowly after the blood draw.

Rest a while: You should stay seated or recumbent for a few minutes. Drinking water before hand to hydrate yourself may help too.