How to unclog a sink

There are two basic types of plungers, the cup plunger and the flange plunger. For working in sinks, lavatories and tubs you want to use the cup plunger. The flange plunger is specially shaped for toilet use. Also, for sanitary reasons you don't want to mix the two.

• First, if this is a lavatory sink, seal the sink overflow outlet found at the top of the sink bowl. Use a piece of duct tape or a wet rag or washcloth stuffed into the holes. NOTE: If you have just tried the boiling water approach you may have to add some cold water to cool the water in the sink down because the very hot water now in the sink may condensate on the porcelain around the overflow, preventing the duct tape from sticking until the porcelain cools off.
• If a kitchen sink or similar, remove the basket strainer.
• If a lavatory sink, remove the drain stopper from the sink. Try lifting it out or turning and lifting.
• Fill the sink bowl halfway with water.
• Place the cup plunger over the drain hole and make sure it has a good seal against the sink surface.
• Using quick, sharp plunges, attempt to clear the obstruction with the plunger and check for drainage.
• Once cleared, remove the overflow outlet seal and replace the drain stopper or sink basket.
• If the clog did not clear, proceed to the next step.

(via About.com)