Drains in the kitchen and bathroom often times start to have an unpleasant odor as a result of various things going down the drain. Get rid of those items and more importantly get rid of the bad smell by flushing these drains with hot water, lemon juice, or other clean solutions to cut the odor.
After getting the sink wet, sprinkle baking soda on the sides and bottom and let it sit for a few minutes. Then scrub the sink using a scrubbing brush. Rinse the sink when you are done.
Keep the drain clean and smelling good by mixing water with various mixtures. You can use a baking soda and white vinegar mixture with water. You can also use a salt and water mixture to cut down on any grease that is in the drain. You can even try a hydrogen peroxide and water mixture as well.
Here is a great video from Howcast on these various approaches.