You probably empty the trash every week and replace the trash bags as necessary. But over time, the trash bags make leak and cause the trash can to get dirty. Over time this can create unpleasant odors and bacteria. It is a good idea to periodically take the trash cans and wash them down on the inside and outside. This can clean any old stains, crumbs, and food that made its way to the bottom or sides of the rubber cans.
Wash the inside of your garbage can with warm water, soap, and a scrub brush. You might also want to spray inside the can with some air freshener. Here is a quick video from eHow.