Heat the magnet above its Curie temperature
Strike the magnet repeatedly with a hammer
Drop the magnet from a height onto a hard surface
Place the magnet inside a strong alternating magnetic field
Use an AC demagnetizer or degaussing coil
Slowly move the magnet away from an energized coil while the current is on
Expose the magnet to strong opposing magnetic fields
Store the magnet improperly near strong magnets until it weakens
Apply mechanical vibration while heating or in a changing magnetic field