My method is very old and crude, Peter. It is much more suitable before you put the new belt on, though. You simply put a wedge-shaped object, such as the end of the right sized flat-blade screwdriver, between the belt and the pulley on the side where the belt is moving into the pulley as you unscrew the nut. Hold the screwdriver handle with one hand, and the spanner with the other, and undo the nut. This will only work if the nut is not going to put up much of a fight, and you don't mind damaging the belt through excess tension (which is why I'd do it before replacing the belt). If it is really rusted up, however, after a long soak in Penetrene I wrap something around the pulley and hold it with a Stillson wrench. The limitation with that method is that if a whole lot of torque is required, you'll end up having to remove the wrapping, and the pulley will be first scarred, then if necessary crushed, by the Stillson. You'll get it off though, or twist off the shaft.
