Too much is never enough!
If something is really stuck, I use a 4 ft length of heavy gauge copper pipe that slides over an 18" Craftsman 1/2" drive breaker bar with a 19mm socket (just like the one shown in Husker's photo above).
Take your weight x 4 and that is how many ft-lbs will be generated by just using your weight.
Here is a pic of this tool on the Boxster (Note: this is only for REMOVAL - I would never use this to tighten a lug bolt - use a proper torque wrench!)