Does your sink have a rubber thingee in the drain? Oftentimes the undersides of these become all gummed up with fat, and other debris, and then the bacteria takes over. Get a brush (toilet brush type) and push it into the drain, and scrub the underside of the rubber thing if there is one. Better yet, if there is one, pull it out and soak it in bleach. Lemons will freshen the drain as others have said.