There are a number of ways to unclog a drain, depending on the nature and the severity of the clog. As a general rule, it is a good idea to regularly scour drains so that clogs are not allowed to develop, and in a household with young children who might be inclined to drop things into the drains, using a strainer in the sink to catch items which could cause clogs is advisable. Strainers are especially useful in kitchen sinks to trap chunks of food and other materials before they enter the drain.
If your drain is clogged so much that it is not draining at all, you have several options. The first is to remove the drain trap or u-bend in the pipe, which is where clogs often form. In order to do this, you should turn off the water supply and get a large tub to catch the water which will spill out when you remove the drain trap. Once you have removed the trap, you can clear out the clog or replace the section of pipe if it is badly damaged.
If you are not inclined to disassemble your sink to unclog a drain, you can also use a snake, a specialized tool designed for clearing clogged drains. Snakes can be purchased from many hardware stores, and some stores rent out a snake to members of the community who do not wish to purchase a snake for a single clogged drain. Snakes consist of coils of cable topped with a wedge-like head which can be forced into the drain and pushed through to the clog before being pulled back. Be aware that using a snake can get extremely messy, as the snake will pull out a variety of substances from the lining of the pipe.
Mild clogs can sometimes be resolved with the use of a plunger to clear the clog. To unclog a drain with a plunger, bail out any standing water, cover any other outflow openings attached to the drain, as in the case of a second sink, and then fit the plunger over the drain opening to make a tight seal, and pump the handle of the plunger up and down briskly. When you plunge a drain, some splashback may occur, so brace yourself. If the clog is made from reasonably soft material, the plunging should push it out of the way, allowing the drain to flow freely again. If you need to unclog a drain in the shower or toilet, be aware that the shower and toilet plumbing are sometimes linked, so fluids may back up into either fixture as you work on the drain.
If you notice that a drain is draining more slowly than usual, you should take action to clear the drain before you have to unclog it. Chemical drain cleaners can be used to scour away substances which might cause clogs, and you can also dump a solution of equal parts vinegar and baking soda down the drain, following with a chaser of boiling water. Baking soda will also freshen a drain, and it can be used in routine drain maintenance to keep drains clear and prevent odors.