One problem with using a plunger in a bathroom sink is that the plunger can not create a true seal because of the overflow opening. Before plunging, cover the overflow with a piece of tape or hold a cloth against it. This will allow the plunger to do it's work efficiently.
Likewise in the kitchen, if you have double sinks, always put the stopper in the other sink so that you can create a suction in the pipe. Harlean from Arkansas