Once a month empty a tray of ice cubes into the sink and turn on the garbage disposal to grind up the ice cubes. After the ice cubes are ground into a slushy mixture, rinse with a light stream of cold water. The ground ice will clog the drain line and water will back up into the sink, however, after a few minutes the ice will melt away and the water will drain. Grinding ice helps to sharpen the shredding blades and remove the accumulation of food waste.
If you have a removable drain baffle, remove it and clean the surface thoroughly with an old toothbrush, water, and detergent. Be sure to place the baffle back in its original location prior to operating the garbage disposal.
Place a half cup of baking soda and one cup of vinegar in the grind chamber. Allow the mixture to foam and fizz while you boil a pot of water. Once the water has boiled, pour it down the drain. In lieu of baking soda and vinegar, you can also mix one gallon of water with one tablespoon of liquid chlorine bleach and slowly pour the mixture into the grind chamber. Allow it to sit for several minutes and then flush with cold water.
These mixtures will sanitize the garbage disposal and remove any foul odor. If you would prefer to not experience the smell of bleach or vinegar emanating from the garbage disposal you can freshen the smell with citrus rinds. Lemon, lime, orange, and grapefruit rinds can be placed in the garbage disposal and ground-up to provide a fresh citrus scent.
You can also use Lemon-Scented Garbage Disposal Cleaner and Deodorizer, which I find equally as effective and pleasant.