Drink loads of water beforehand, you don't want thick blood and water will help you if you have low blood pressure and feel faint.
When I'm waiting to be called I sometimes ask to use the washroom. Run hot water over your hands, it gets the circulation going. Your vein will be much easier to find when you do this, no multiple pokes!!!
Ask to lie down if you pass out most of the time, my dh has to do this, makes me smile. Such a man until the needle comes out!
When she gets the elastic around your arm now is the time to find a place on the opposite wall with your eyes and practice your Pilates or labour breathing. Pump your hand to get the vein working, stare a hole in the wall and breath in deeply through the nose (BIG breath -flare those nostrils) and out through your mouth until lungs are empty. Do this until the needle is done.
You most likely pass out if you don't breath deeply because your little ticker starts going a million beats per minute (anxiety does this to the best of us) and your breath gets too shallow. Just picture yourself in labour, if you can give birth baby, you can give blood. Deep breaths! It gets easier the more you focus on your breathing. You forget about the needle running the breathing through your head. Speak to yourself 'breath in breath out'.