Try the steps below in order. They are organized from simple to complex. If you are unsuccessful, then you can call a plumber. Caustic drain cleaners seldom dissolve tough clogs. They can actually become a hindrance if the clog remains and you need to bail out the sink before trying alternative methods to clear your drain.
Preventive measures are the best alternatives.
To view a multimedia demonstration of this tutorial, click here . The demonstration requires Macromedia Flash 4 player. If you don't already have it, click here to get it. The first thing you should try is a little preventive maintenance. Unlike the kitchen or utility sink which both get purged clean by hot water, your bathroom sink needs a good flushing now and then.