Check your coils on your refrigerator and clean them once every 6 months. More if you have pets inside. Lint and hair build up on the coils can cause your unit to not cool properly. The coils ae the only way the unit has to dissipate heat and to keep it running properly.
Clean any spills inside and out of the unit immediately. Spills that set for a while can cause a stain on the interior that cannot be removed. Also, the area where the door seals close needs to be clean and free of any spills to help maintain proper seal. Remember refrigerators were designed to cool the air inside the refrigerator and not out. If your unit is not sealing properly then check the door seals. Cracks, splits, torn and missing pieces will not seal properly and cause warm air to enter thus causing you to lose the proper temperature needed to keep your food fresh. If door seals are bad then replace them.