excellent advice!! I second the advice about not slapping the skin. I was a phlebotomist for years before going into nursing and I NEVER slapped anyone's arms/hands and nearly always got blood on the first stick. And, a warm compress for 5 - 10 minutes will get the blood circulating to those cold extremities.
OP, you will "get it". As everyone has said, it just takes practice. If you have friends, relatives, spouse or total strangers that you know that are willing, practice on them. Pay them, bribe them, ply them with good food and wine.
Believe me, once you "get it" it will become second nature.