somewhere in the past someone said use 1/2 a credit card depth as a guide for setting your blade depth . . . .This is just plain wrong - you want to cut thru vinyl that is just a few mills thick and not go into the backing paper. when properly set you will barely see the blade - take the holder out of the machine and adjust the blade so it just cuts thru the vinyl and not into the backing paper - put the holder back into the cutter and adjust your pressure.  Somehow this myth of 1/2 a credit card just won't die . . . . 

If you use a premium blade make sure to start with too little blade and too little pressure until you get it set.  premium blades have harder tips to make them last longer - this makes them more brittle and will break if you punch thru the paper into the cutting strip - when the blade makes a turn.   This has been a public service announcement.

Edited 12/12/15 to add skeeters instructions and the link to graphtecs instructions in the first post of the thread

From Mz. skeeter and the method used most for accurate blade depth
  To start with, you should set your blade depth correctly, by taking the blade holder out of the machine, and firmly cut across a piece of scrap vinyl, you will be cutting. You should only be cutting the vinyl and barely a mark on wax paper backing, Adjust blade to get there, Then put the blade holder back in machine, and use the force of the machine to get there, same results, only cutting the vinyl and barely a mark in wax paper backing.

from graphtec on blade depth, offset, etc