What size do you need? Standard mattress dimensions in the U.S. are: twin, 39 inches by 75 inches; double or full, 54 inches by 75 inches; queen, 60 inches by 80 inches; standard king, 76 inches by 80 inches; and California king, 72 inches by 84 inches. Some high-end mattresses don't come in sizes smaller than a queen, so make sure the mattress you try in the store is available in the size you want.
How tall do you want your bed? Mattresses seem to be getting thicker and taller all the time -- a really thick pillow top can be so tall that you'll need extra-deep fitted sheets for them to fit on the bed. However, platform beds can take your high-profile mattress down a notch, to make it easier to crawl in and out of bed, while box springs, or foundations, can raise it up if you prefer a lofty look.
How do you sleep? People who normally sleep on their sides may prefer a softer mattress than back or stomach sleepers. However, a mattress of medium firmness is usually comfortable for everyone.
Do you overheat at night? Foam mattresses retain more body heat than innersprings and air mattresses. A latex foam mattress may provide better ventilation than memory foam.
Will you share the mattress? If your partner is a restless sleeper, you may prefer a foam mattress or an innerspring with pocketed coils, since these are generally better at isolating motion. On the other hand, the bounciness of an innerspring may make it more suitable for sex than a foam mattress that conforms to the body. If you strongly prefer a firm mattress while your partner prefers a soft one or vice versa, your best option may be a dual-chambered air mattress, which can be adjusted to a different level of firmness on each side.
Do you need a box spring? All beds need some kind of support, whether it's a platform, closely spaced slats or a box spring. Box springs have undergone quite a few changes in the past decade or so, and often do not even have springs in them any longer, since mattresses have become more springy and thicker. While traditional box springs still exist, they are largely being replaced with "foundations," which and are sturdy boxes made from wood or metal. Regardless of which type of mattress you choose, you will need to support it via a box spring or foundation, by the use of a platform bed, or with a piece of plywood or very closely spaced slats on your bed frame. Mattresses can also be placed directly on the floor, although most people don't find that very comfortable for getting in and out of bed. Most manufacturers also make box springs and foundations in "low-profile" styles so that the mattress is supported, but you don't have to climb too high to get in bed.
When surfing the larger manufacturer's website to learn about their mattresses, just ignore whatever price they have posted there -- that's probably not even close to what you'll actually pay. The "suggested retail price" on mattresses tend to be exceptionally high, as models are often priced to be sold at discounts of 50 percent or more. And don't ever be afraid to haggle when you're in the mattress store. If they know you're serious about making a purchase, salespeople may find a way to sweeten the deal rather than watch you walk out the door. Even if you can't knock down the price, a little bit of negotiation could land you a free foundation or a good deal on a frame or headboard.
Visit manufacturers' websites. Before shopping, browse through the different lines to see which features they have, and print out details about the models you're interested in. This gives you a good starting point when you get to the store.
Start at the bottom of the price scale and work up. Mattresses vary tremendously in price, and all but the very cheapest are likely to be sturdy.
Rest on the mattress for at least 15 minutes. When trying out mattresses in the store, spend time on your back and both sides as well as your stomach, if that's one of your normal sleeping positions. If you sleep with a partner, try out mattresses together.
Check the firmness. If you have difficulty rolling over, the bed is probably too soft. On the other hand, if you can easily slide your hand under the small of your back while lying on your back -- or if you feel pressure on your hips and shoulders -- it's probably too firm.
Ask to see a cross-section of the mattress. Look at details like the coil size and density of an innerspring mattress, and the foam density and size of ventilation holes on a foam mattress.
For an innerspring mattress, consider a new box spring. Some sources say an old box spring can be kept if it's in good condition, but others say a new one will promote comfort and prolong the life of the mattress. Also, using an old foundation might void the mattress's warranty.
For a memory foam mattress, be sure you have proper support too. While most memory foam mattress manufacturers do not recommend a box spring, you will still need some sort of platform and frame to set it on, unless you're planning to place it directly on the floor.
Look for a comfort guarantee. Many retailers - both online and brick-and-mortar -- will give you a trial period of two weeks to three months on a new mattress. Each store has different rules, and there may be a fee for returns or exchanges. Make sure to get the details in writing, or be sure it's clearly spelled out on the website if you use an online-only store
Ask about disposal of your old mattress. Some stores offer free disposal; others will charge a fee. In some areas, mattresses and box springs can be left curbside for free pickup by local sanitation services, but check first regarding any special regulations -- such as specific scheduling/days or sealing in protective wraps to prevent the spread of bedbugs.
Read the fine print. Make sure the sales contract specifies that the store can't deliver a substitute brand or style if the one you want is out of stock. When your mattress arrives, inspect it to make sure it's the right one and that it's undamaged.