The second rule is that scratching posts should be absolutely secure. Cats think there's nothing worse than having the post wobble or fall over when they're in the middle of a good stretch. The third rule is, the post has to be covered with the right kind of stuff. Tightly wrapped, uninteresting carpet is out, and burlap and other easily shreddable things are in. Cats prefer vertically oriented fibers, as this orientation lends itself to shredding.
Many owners change a scratching post when it is old and tattered.
This is dead wrong, as shredding indicate frequent use and that is exactly what you want. Half the fun cats experience from a scratching post derives from getting their claws stuck in the material and leaving wispy threads as testimony of their erstwhile presence. More than one post is usually preferred, perhaps one for each high-traffic area of the house. Different kinds of scratching posts will provide different challenges, so owners can profitably get quite creative. It may be awkward for the humans to have to circumnavigate an assortment of burlapwrapped posts in the center of living areas, especially at night when the lights are out, but fear not, these obstacles can be inched to more-convenient locations over time.