Summarize this article:

Storing your mop while it is still wet encourages mold and bacteria to grow. Let your mop head dry completely before you store it so that your mop head will last longer. Dry mops, sponge mops, and other mops with flat heads should be stored with the head up. This keeps their heads off the floor and away from dust and debris while not in use. It also helps them better maintain their shape. Traditional wet mops should be stored in a way that allows their strings or strips to hang down naturally. If possible, the best way to do this is to hang the head off of a hook or rack on the wall, separate from the handle. If you cannot store your wet mop head separate from the handle, store the mop with the head up. This may cause some misshaping, but it is more sanitary and will help your mop last longer. To help avoid mold and mildew growing on your mop head, keep it in a dry place. A closet or another indoor space generally works just fine. Garages that aren’t temperature-controlled and laundry rooms aren’t always the best idea, though, as these can get humid. Exactly when you need to replace your mop will depend upon how often you use it and what you use it for. In general, though, a mop head will last about three months. If your mop smells even after disinfecting or if it has continued problems with mold growth, replace it immediately.

Summary:
Let your mop dry completely before storing it. Store dry mops and sponge mops with their heads up. Store string and strip wet mop heads separate from the handle. Keep your mop in a dry place. Replace your mop as necessary.