First, when shopping for a new mattress, make sure to take your time. You're about to make a big purchase, so don't rush into it. To get the best deal and bring home a quality mattress that works for you, you'll want to compare prices and try out the products (yes, that means lying on several beds on the showroom floor).
Before you go to the store, try to decide what kind of mattress you want. The most common type is an innerspring. As the name suggests, innerspring mattresses have coils inside them that provide support while you sleep. They come in a variety of styles, options and price ranges.
Foam mattresses, on the other hand, rely on foam rather than springs for support and cushioning, they conform to your individual shape. They're popular bedding options but are more expensive than innersprings, and some foam owners have complained that the beds retain too much heat and can make you hot while you sleep. However, many people swear by foam mattresses and wouldn't think of switching back to an innerspring.
If you and your partner can't seem to agree on a mattress, you may want to consider one with adjustable firmness levels. Mattresses containing air chambers can inflate or deflate -- allowing you to adjust the firmness of each side at any moment. Like foam options, these air mattresses tend to be more expensive than traditional innersprings, though they remain a popular alternative.
Regardless what kind of mattress you're looking for, you should decide how much you're willing to spend before you walk into a store. Just like when you purchase a car, if you go in with a spending limit in mind, it's less likely a salesperson will convince you to buy something you can't afford. Stick to your budget, and don't let salespeople bully you into spending more than you want.
Once you're in the store, start trying out products. You may feel silly, but lie down on the bed in your usual sleeping position and see how it feels. Consumer Reports recommends spending 15 minutes on each mattress that you're interested in purchasing. If you tend to change sleeping positions during the night, consider spending some time on your back, side and stomach to see how each one feels.