Update: I have found the iSonic Ultrasonic Cleaner to be one of the best methods of cleaning all snoring devices.
The human mouth houses between 800 and 1,000 strains of bacteria, which makes cleaning your mouthpiece after wearing it very important. If you stored your Zyppah without properly cleaning it, the device probably wouldn't actually start smelling for a few days, but it would definitely be unsanitary and a breeding ground for bad bacteria by the second time you put it in your mouth.
Zyppah should be cleaned with a soft-bristled toothbrush and mouthwash. Just pour a little mouthwash into the bottle cap or some other small container and dip the brush in before you scrub the device. Be sure to get the small space between the upper and lower pieces. Rinse thoroughly, and allow to air dry completely before putting it into any type of storage container. Learn more about Zyppah in general in my Zyppah Review.
Zyppah recommends that you give the device a deep cleaning at least once a week using Efferdent, Polident, or some other type of effervescent tablet. Allow the tablet to dissolve in the water before adding your Zyppah. Soak for 5 to 10 minutes, rinse it thoroughly, and air dry.
Zyppah also offers a product called SNAP, which is a cleaner and sanitizer in one. If you use this product, you will not have to use mouthwash or denture tablets. SNAP is available in 30-day supply boxes. If you sign up for the replenishment program that automatically sends you a reorder every 30 days, you save $5 off the price and $4 off shipping charges.