Mops and brooms are used to remove clutter and dirt from the house. Feng shui believes they carry part of the negative energy that comes into the home. Keeping them outside would be ideal but is not necessarily practical. Keep them in a cupboard in an upside-down position to help to block the negative energy.
Try to make sure that the cupboard you store a mop and broom in is not in the room you eat in, as the dining room represents food and prosperity, and you don't want that swept away. Also, a broom or mop should not live in the front room of your home, as you don't want the positive energy that arrives to be swept away.