Not that I'd especially recommend this plan, but my general rule of thumb is I take either a big knife or some sissors to the blister. Usually I can cut it open, let it drain, and then move on.
A couple nights, however, I was trying this method on a small but deep blood blister on the very heel of my foot. I was using the sissors from a Letherman knife and having a hard time getting it open. Finally I cut it open and blood spurtted out all over my hand like I had been shot in the head or something. The wound did not bleed any further, but it was pretty crazy none the less.
One last thing: blisters suck