Put 1/2 cup baking soda down drain. Do not run water (yet). Pour vinegar down drain. Can heat vinegar. May wish to use a plunger and/or let sit overnight. Follow with boiling hot water. Do regularly, once or twice a month, as a preventative measure.

For a grease clog:
Pour cup of baking soda along with a cup of salt into drain. Then pour kettle of boiling water down drain.