Baking soda and vinegar are always in the kitchen. This really works, the only caveat being that you have to do it more often. But then you'll save yourself from using the highly toxic, Drain-o stuff that really doesn't do a perfect job anyway AND it contaminates your home. Of course, the best thing to unclog a drain is a plumbing snake, but that's for another day.
Baking soda and vinegar produce a remarkable chemical reaction that will clear out whatever's in the way when used with hot water and a plunger. The basic directions are...
1. Get as much water out of the area as possible
2. Pour 1/2 cup of baking soda down the drain
3. Follow with 1 cup of HOT white vinegar
4. Wait 10 minutes or until it stops bubbling and wash with boiling water
5. Use a plunger if necessary
General consensus seems to be that the vinegar and baking soda can be left in for any length of time, and that the hot water really makes a difference (which was a part we didn't know).
Here are some great links that will tell you more: