Know it's time. Lay a yardstick on your mattress and see if it touches all the way down. If there is an impression greater than 1 1/2", you are not being supported properly.
Choose a comfort level. You want a well-built mattress that gives you great support and good value for your money. Choose from extra firm, traditional firm, cushiony and plush or extra plush with a pillow top.
Take a test drive. Don't just sit on the mattress-lie down with your spouse for at least 10 to 15 minutes. Be sure to toss and turn, just like you do at home.
Pick a size. Think what you will be using the mattress for. Plan ahead for partners, children, pets, etc.
Stick to a budget. Determine what your budget will afford and stick to it. A higher quality mattress is usually bought at a higher price. However, price isn't everything. Focus on the models that suit your needs best!