You Brush your teeth every day, so doesn't it make sense to clean your retainer every day too?
Your retainer builds up problem-causing bacteria just like your teeth do. Make sure to clean it every day!
Use a toothbrush and some toothpaste or a small amount of non-abrasive soap (such as Dawn) to brush all surfaces inside and outside of your retainer. If you have a clear, Invisible-Type (Essix-style) retainer, or NightGuard, be sure to use cool water as these types of retainers are heat sensitive! Make sure to rinse well and you're good to go for that healthy smile!