Money Is No Object
You should set a budget for yourself, but remember that mattresses are expensive and your quality of sleep is priceless. A good night's sleep keeps us healthy, makes us more productive and puts us in a good mood; keep these things in mind when you come across a mattress you like that has a price tag you don't -- and then negotiate for a price you do like. Typically, sales people are willing to bargain with you if you bring it up.
Mattresses are expensive and we want them to last forever, but they don't. Your mattress should be replaced every seven to 10 years. Flipping and rotating helps you get good mileage out of a mattress -- every three months is good for maintenance. If you're doing this but you're waking up with aches and pains, you should consider getting a new mattress even if you haven't had it for seven years.
Elements of Comfort
Pay attention to how you sleep in your bed -- not everyone sleeps in the fetal position and not everyone needs the same level of support from a mattress, so it's important to test out mattresses thoroughly before pulling out your wallet. Spend a significant time with each mattress that piques your interest. Lie down in your normal sleeping position to test if you're able to relax. You should give yourself about 15 minutes with each mattress.
A lot of us share beds with people who have different mattress comfort needs. Make sure to bring your partner with you when you're shopping so both of you can test out each mattress. Make sure you test the mattresses at the same time so you and your partner can feel how each of you is affected by the other's movements. This helps you weigh the pros and cons for specific areas of focus for both of you.
Box springs don't seem that important, but they're critical for getting the most out of your mattress as well as give it support. Box springs wear out along with your mattress the way your tires on your car wear out at the same rate, so it's important to replace both the mattress and the box spring at the same time.
Tricks Of The Trade
There are little add-ons that mattress dealers use to sell mattress that don't add to their value at all. Look out for no-flip mattresses, wrapped coils or special coverings -- they're all gimmicks designed to do nothing but make your mattress more expensive.