Mold, bacteria, strep, and staph easily invade mouth guards during normal use.
When not in use, store it in a case
The case will extend the life of your mouthguard and prevent it from getting damaged. And especially if you carry your mouth guard to and from practice in a gym bag, the case will also prevent it from getting dirty from bacteria.
The mouth guard should be stored in a perforated case to allow for air circulation but firm enough to prevent damage. Make sure the mouth guard (and case!) is dry before closing the case.
After each use, clean thoroughly
A simple mouthguard cleaning procedure won't take much time or effort. Scrub it gently with your toothbrush and warm water before bedtime, and let it dry naturally.