Before and during mattress shopping, gather information. Learn about different sleep systems. Read about foam mattresses, air filled mattresses, water beds, inner spring interlocking coil and innerspring independent coil. Research different brands of mattress manufacturers using consumer rating websites or publications; ask people you know how they enjoy their mattresses.
Remember that excessive firmness is not always the best choice. Some softness and give is better for spine health. Also, choose the right mattress size for the height of the person using it. Look for numbers of coils, high numbers of interconnecting wires on an interlocking mattress and sturdy edge materials. Look for a 10-year warranty at least, and realize that mattress warranties are prorated the same as automobile tires, you will only be credited for the unused portion of the warranty time frame.
Also similar to cars, mattress prices are negotiable. Take time to lie on a mattress for more than a few seconds, since that is the ultimate test. Bring a partner with you to shop if you will be sharing the bed. Write down features and prices in detail, since stores often have different names for mattress models to confuse customers. Shop first -- don't buy on the first trip.