We have this trouble from time to time. Is it your kitchen sink? I use white vinegar, borax, and boiling water. put the borax down, wait a few minutes, give it a big shot of vinegar. Then start your water. When it boils, pour it down.
If that doesn't work, I have used this product before. It really does work, and is worth the xtra money. I wouldn't touch drano with a 50 foot pole, sorry, Robert!

I

my watershed too much!
http://www.ecos.com/pages/earthenzymes.html
Several of my local stores carry it, so I have used it personally with success. It eats the goo ball the pp was talking about. Or the hair, if it's your bathroom sink or tub.
One thing to consider, as well, is whether or not it is a symptom of a greater problem, such as tree roots in your sewer lines. We have this problem often. You can purchase a completely natural enzyme root killer called root x. In my state, (PA?) you cannot purchase it from a store, as it is a controlled substance (?) and you need a permit. We have a friend who works at public works (who we had to come out and drill a 3 in root out of our sewer line) who advised it is the safest thing on the market and to purchase it off the internet. Drano is toxic to watersheds. This is not. Here's a link:
http://www.jsplumber.com/
Totally worth the price, after the second time we called a plumber in the middle of the night! We use it about every six months or so.
Good luck!