When my drains start to smell, I pour in some baking soda, followed by a small amount of vinegar. It will foam up; I let it sit awhile, then flush it with water.

Run a lot of HOT water after the baking soda/vinegar has done its' job. The hot water will force anything bad down the drain and dissolve any fats in the line.
My plumber taught me this.

I do the above but follow with a boiling tea kettle full of water. :) It will work! Same is perfect for the garbage disposal.

I do much the same, but first I use the vinegar to clean my coffee pot -- then put soda down the drain, followed by the hot vinegar, then hot water. Just did this yesterday because we thought the coffee started tasting "off" -- so now the coffee pot AND the drains are clean!