Keep your retainer clean by brushing it gently with a toothbrush and toothpaste. You may also soak it in denture cleaner as instructed by your orthodontist. Do not put your retainer in boiling water or the dishwasher.
Keeping your retainer in good shape will help it do its job. Put your retainer in its case only when it's not in your mouth so that it does not get lost or broken. Always keep your retainer case with you. Drink only water when you wear your retainer, as other beverages can get caught between the retainer and your teeth, leading to decay.
You can't remove a fixed retainer while eating, hence there is a chance for periodontal issues due to poor oral health measures. However, there's a greater chance of retaining all the benefits your braces provided because fixed retainers constantly keeping your teeth stable. Brush after each food, floss once in a day diligently, perhaps using an interproximal toothbrush to remove the plaque and tartar prone to build up around fixed retainers.