Do wash it every night with a toothbrush and toothpaste (this may not apply for all retainers so please ask your dentist about this).
Do put your retainers in its case when you're not using it.
Do disinfect your retainers once a week using a brand name that you should be able to find at a pharmacy.
Don't leave it in your bag or your pocket; it can break.
Don't play around with the plastic/metal; it can damage it and therefore not work effectively or worsen your teeth.
Don't wash it in hot water as it may cause the shape of your retainer mold to change. Instead use cold or lukewarm water. (Just to be safe I would go with cold).
Other tips and suggestions?
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A Girl's Guide
RandomTo put it simply, a girl's life isn't complete without this book. WARNING: You may suffer minor mental dysfunction and shock as this is probably the best girl guide known to mankind.