Brass is water-friendly & can be worn in water. However, moisture accelerates oxidation & makes it tarnish faster. So it is best to avoid excessive moisture 

Proper Storage: Store jewelry in pouch or jewelry box when not in use to slow down oxidation.

Cleaning: Easily restore shine using a specialized metal polish (such as "twinkle" ) or a gentle DIY method using equal parts of lemon juice & baking soda. ( use soft cloth/toothbrush )