The general rule of thumb is that a good-quality mattress will last between eight and ten years. But because each person and each mattress is different this is a rough estimate. The quality of the mattress, how it is used and how often, and the effort you put into keeping it in good shape and all combine to determine its life span. A mattress that has a good number of coils; tight quilting; and even, sturdy seams will withstand daily wear much better than a lower-quality product. Obviously, a mattress you sleep on every night will not last as long as one that is kept in a guest bedroom and slept on only occasionally. If you also use your bed as a place to read, write, watch television, and so on, it will wear out even more quickly. The shape and weight of your body will cause your mattress to settle in certain spots over time.

The telltale signs that it's time for a new mattress are hard to miss: If your bedding sags in spots or looks especially worn, if it creaks, if it feels bumpy or otherwise uncomfortable to lie on, and especially if it affects your sleep or leaves you in pain, then you should start shopping for a new one.

In general, people who sleep on their backs tend to prefer firmer mattresses, while those who sleep on their sides or stomachs favour softer ones.

The basic factors to consider when choosing a mattress are size, comfort, support, and durability.

• The ideal size depends on who will be sleeping in the bed; the mattress should measure at least 6 inches longer than the height of its occupant(s).

• The only way to test for comfort and support is to actually lie on the mattress before purchasing it. First, stretch out on your back and then assume your favoured sleeping position. Have your partner do the same. The mattress should support your body at all points and keep your spine in its natural alignment. You may want to consider buying a mattress that is firmer than you are accustomed to, as it will soften with use; if the mattress is too firm, however, it will not support your body evenly and may cause you discomfort. The number of coils in a mattress won't necessarily determine how firm the mattress is, because placement of the coils is equally important as number. The padding also contributes to the firmness. But a good way to judge the coils is the thickness -- they should be 13 gauge or lower. (The salesperson or manufacturer may be able to give you that information.) Lower gauges and less padding will make the mattress firmer.

Excerpted from Martha Stewart's Homekeeping Handbook by Martha Stewart. Copyright 2006 by Martha Stewart. Excerpted with permission by Clarkson Potter. All rights reserved. No part of this excerpt may be reproduced without permission in writing from the publisher.

• To determine durability, carefully inspect the mattress: The seams should be straight and the quilting should be tight and uniform. There should be no broken threads and the mattress should have sturdy handles for turning. Each innerspring coil should be turned at least five times; tighter coiled will make the mattress more supportive and long-lasting.

Most commercially manufactured mattresses contain flame-retardant chemicals, mold and mildew inhibitors, and stain guards to protect the exterior. Some mattresses may also contain formaldehyde, a known human carcinogen. These substances will "off gas," or give off fumes, over time. If you are sensitive to these agents, consider a specialty-mattress maker who uses chemical-free materials.

If you happen to have a vintage bed that will not accommodate a standard-size mattress, check with an antique dealer who sells beds. He or she may be able to recommend a custom manufacturer.

Warranties are usually activated automatically from the day you purchase your new mattress. Often consumers will need only their original store receipt and the law label from the mattress to prove their warranty, but be sure to check with your retailer when you purchase your mattress.

Excerpted from Martha Stewart's Homekeeping Handbook by Martha Stewart. Copyright 2006 by Martha Stewart. Excerpted with permission by Clarkson Potter. All rights reserved. No part of this excerpt may be reproduced without permission in writing from the publisher.