First of all you should check the unpleasant odour and if there is any then you will have to remove the retainer and clean it properly without any delay. There are many cleansing liquids available in the market. Pick the one you like and wash your appliance according to the instructions.
Make a routine to clean your retainer after every meal. You should brush and floss immediately after eating something. Similarly, you can also clean up your retainer by just taking a bit of mouthwash and swish around your mouth.
You can keep your retainer clean just with a toothbrush. All you need to do is to rub your brush thoroughly on both sides and around the wires. However, you should consult your dentist before doing this as sometimes retainers are made of sensitive materials.
Be watchful while having a retainer in your mouth. You should keep an eye on calcium build up. In the market, there are many products for the removal of calcium but you can also do it at home. For that, you will have to get one cup of white vinegar and soak your retainer in the cup. When the calcium becomes soft then it can be brushed away easily.
In recent years, a new product has been introduced to remove tartar, plaque and bacteria from your retainer. These products do that with the help of sonic vibrations. All you need to do is to place your retainer in a small machine with bit of cleansing liquid and let the sonic vibrations do their work for 15 minutes.
If it is becoming difficult for you to follow a proper routine then soak your retainer at least once a week. However, make sure you wash it properly before using it again.