Store the bulk of your African black soap wrapped up preferably in a paper back or similar and place in a dark place away from any kind of heat or sunlight. You can also store it in an airtight container and place in a dark area.
In the shower store the black soap in a dry soap dish or surface. Avoid leaving your black soap in water or moisture if you want your soap to last. Leaving the soap in water or moisture will dissolve it or make it every soft.
You can also store your soap wrapped in a towel. Depending on the size of your soap it will be wise the cut them into the size you want before storing. This will make it easier for you to store. Black soap will get harder with time so cutting it before storing saves you time.