Mattress quality is difficult to judge in the showroom, so do your product research before you visit one. Here are some basic guidelines. As a general rule, it's best to buy the highest-quality set you can afford.
High quality latex foam cushioning is longer lasting (loses resiliency slower) than polyurethane foam cushioning.
Springs and Thickness
Extra-thick mattresses require extra-long coil springs–not simply short springs that have been stretched. Otherwise your mattress will lose resiliency quickly. Compare spring samples before you buy. Often the number of springs in a mattress is an indication of quality, but not always. Nor is thick mattress padding an indicator of durability.
If you sleep with a partner, opt for a king- or queen-size set to allow each sleeper extra space and so weight is not concentrated in the center of the mattress, where it can cause wear to the coil springs.
Tip: Never use a new mattress with an old box-spring. Mattress-and-box-spring sets are designed to work in unison (the springs align), which adds life to your mattress and ensures maximum comfort. In fact, experts claim that using a mattress with a mismatched box spring can reduce its life by 50 percent. It may also void the mattress warranty.
Plus: 4 Secrets to Make Your Mattress Last Longer
How to Buy a Mattress, Source:https://www.rd.com/home/improvement/how-to-buy-a-mattress/