I used the pencil trick, just bought a pack of #2 pencils from Wal-Mart for like 98 cents. I did it right the first time, because I've had my multiplier unlocked for the better part of 3 months now. The thermal grease does nothing to mine, because I only applied it to the actual die on the processor, and the L1 bridges are seperate of that.
Some people have said it has rubbed off, but I really don't see how, unless they take the processor on and off constantly. If you are like me, and only get inside your case to install a component every once and a while, then just use the pencil. Ease of correction makes it the best method if it is your first time doing it.