Store your broom by hanging it up indoors for the best possible care.
Hanging your broom keeps the ends nice and straight, rather than storing it leaning on its fibres. If you have nowhere to hang it you could also just flip the broom upside down and lean it in a corner on its handle.
Storing the broom indoors is important to protect it from climate. This is most important in damp climates. Even covered porches/unheated sheds etc may be too damp to store natural fibre products. In dry climates it is less of a concern, but rainy seasons still happen and also direct sunlight over time could bleach and damage the fibres so we always recommend storing them indoors.
It is okay to clean the broom with a damp cloth & soap if necessary, just make sure it is hung in a well ventilated area to dry, and try not to get the tightly bound areas of the fibre too wet.
If you live in a dry climate and the broom is feeling a bit dried out you can actually dip the lower ends in a bucket of warm water and it will pull some natural moisture back into the raw fibres. Simply hang the broom to drip dry after soaking it.
If you accidentally left it outside and it got a bit of mold on it, a bleach and water solution and a scrub brush should remove minor damage. We've been told the product called "30 Seconds" also works well but haven't personally tried it yet!