My good friend dumps baking powder down her slow drain, then pours vinegar on it. I tried it yesterday and it cleared it out. Sometimes she uses a plunger after the soda/vinegar application too.
Pour boiling water down the drain. Then add a half cup of baking soda and let it sit for a minute. Pour a cup of vinegar and a cup of hot water down the drain. Cover loosely and let it sit for 10-15 minutes. Pour another pot of boiling water down the drain to flush out the clog.
Sometimes it's worth it to call in an expert. When my sink drained slowly, we discovered that the problem was where that drain connected to the main drain in a Y-connection. It involved old pipe, which had to be replaced. Putting clog removal stuff down the drain wouldn't have helped!
I agree with all the above responses, but another one I use is to regularly pour a cup of Dawn down the drain followed by hot water.