Lotta misinformation here. Demagnetizing definitely helps, needs to be done regularly, but especially with a cartridge (MM or MC!) needs to be done carefully.
Anyone in doubt is encouraged to do a little reading on magnetization. Very simply, magnetization happens when a strongly oriented magnetic field is presented and then cut off. This is exactly what happens with music, where the signal is constantly swinging positive to negative. In fact this is exactly how we demagnetize! There is however one very big difference. In demagnetizing the amplitude of the signal is gradually reduced to zero. In music the amplitude is all over the place. Wires are never perfect, they all have areas small or large that can become magnetized. The result is that gradually over time playing music results in lots of isolated little magnetized regions in wire.
Demagnetizing is simply an alternating polarity field that starts out very strong and then smoothly fades out. The strong field assures getting even highly magnetized areas, and the smooth fade takes it all back to neutral, demagnetized.
That's it. It ain't rocket science. It can be done any number of ways- sending a signal through the wire, playing a smoothly fading track, manually holding a powerful demagnetizer near and then moving it smoothly away. All of these work great. Just try them and see.