We've all been there. You're working on your brakes, or installing new shocks and you need some help. Loosening a bolt, you look on the back and the nut is spinning. No problem, you'll just put a wrench on it. The problem is that you don't have an extra hand to hold the wrench on so you can wrap up the brake job.
In many situations like this, you can lodge a wrench into position so that it holds the nut in place.

Just slip the wrench over the nut (a closed wrench works best) and turn the bolt on the other side until the wrench is wedged against something solid. Now you can loosen everything up with ease!