I know this thread was really old but the best place to draw blood is from the tail below the cloacal vent opening. Turn the snake upside down, and use a TB syringe with an apprpriate size needle. The bigger the snake, the longer the needle. Place the needle and syringe 90 degrees and push needle all the way in, until you feel the needle hit the vertebral column. Then pull the plunger and ever so slowly, withdraw the syringe, keeping pressure on the plunger. As soon as blood begins to fill the hub, stop withdrawing the needle but keep the pressure on the plunger. Depending on what tests you want to run will determine the amount and type of containers to use. You can't use the typical small animal purple top tubes or serum tubes. If you contact your local lab, they can supply you with appropriate microtubes to put them in.
It's also really cool to set up a slide and do differentials on them. I got pretty good at that.