Is it a fasting glucose test they are doing? If so eat a sandwich before midnite and drink plenty of clear fluids thats not just water its tea too but do not put milk or sugar in it. And take something sweet with you for afterwards just to get your blood suger back up quick ie some lucozade and chocolate. As said above get them to lie you down first and this should actually make it abit easier for them to get a needle in you.

Its not nice to pass out and you are not the only one I have men passing out alot when we take bloods and if you do pass out on the bed they can continue looking for the vein while your out