"I've been in the furniture and bedding business since birth.... yes birth. (33 years) I hope this info is useful. I know it is long but you do spend 1/3 of your life in bed a good mattress and box spring set is worth the investment. However it doesn't have to break the bank. An excellent quality comfortable name brand set can be had in the neighborhood of $500-750. When looking at a $3000 set one has to ask "what could possibly be in that to make it so expensive???"
When buying a mattress one needs to do some research. Mattress are like anything else. There are a lot of bells and whistles that a person really does not need: Kasmire damask cover, 8 way hand tied box spring, etc. Ask yourself "do I really need this stuff?" Pay attention to the gauge of wire, not just the coil count. And remember that the smaller the # gauge the heavier the wire ie. 13 gauge is better than 15.
When buying a mattress always alway always replace your box spring!!!!! I cannot stress this enough!!!! The set is made to work together. The box springs are located in position to to support the mattress springs. One brand does not match another. Always buy the set!!! Even though your old box spring "looks fine" it has had just as many years of wear and tear as the mattress and is worn out. Without proper support for your new mattress you can expect 50% life out of your mattress. Do not buy a set with a foundation. That is a solid box with no springs. Again 50% mattress life. Always use a bed frame. Preferrably a heavy duty one with proper center support.
Do not buy a mattress from a department store. They are higher priced and are made specially for the stores specifications to meet a price point, meaning they have had things taken out that the local independant's mattress of the same brand will have. Again this is done to reach a desired selling price.
When trying out a mattress it is important to lay fully on the bed. Head to foot. Yes, feet up. Plan to spend a good 10 to 15 minutes laying on the beds that feel good to you. Don't just bounce up and down on your rump on the edge and don't lay sideways across the mattress with your feet dangling down unless this is the way you plan on sleeping!!!
Stick with a name brand. Some are better than others. Do your homework. Higher price does not mean a better product. You may be buying things you do not need. If you are interested in a pillow top mattress, keep in mind that all pillow tops (unless it is a latex topper) will get body impressions. Buy a name brand mattress with a non pro rata warranty of 10 years. ALL MATTRESSES HAVE A LIFE OF 7-10 YEARS ONLY unless they are a solid latex core (no springs at all)
Don't buy at the first store you go to because they claim to have the lowest prices. Remember, the more a store advertises the higher it's costs and the higher the price you pay. Add up all of the free items from one store vs. add on costs at another, ie. free frame, free delivery, free sheets. Most times you will find you pay less at a store that adds these costs on at the end. Remember there is no such thing as free anything. It was added into the cost of the mattress.
Do not buy a "rebuilt" or "reconditioned" mattress. That consists of putting a new cover over the old soiled one. Nothing is done to rebuild it. Once it is worn out it is garbage. Many times these have not even been sterilized and can be contaminated with urine, bugs, mold and meldew. Gosh knows what else. They can be the cause of many unknown health problems.
Check out the 4 "big S" brands. One stands head and shoulders over the rest and it is not the largest company. You want to buy a mattress with a minimum of 312 coils, 480 is better, 540 is best. Much higher than 540 the guage of wire is thinner and the coils are smaller, meaning weaker. The coil count is always quoted in the full size, so when buying a queen size etc. the coil count is actually higher. This is just a standardized way the industry uses for specifcations.
Don't just trust your sales person, they are usually commissioned and out to make the sale. Ask to see the factory specifcations and ask questions if there is something you do not understand. Many times the store will have cut away buns to show you what is inside.
And lastly before I get off my soap box, please, when purchasing a mattress for a child remember that their spines are continually growing and forming. It is important to buy a good set for them, also. It need not be so high in coil count (here a 312 coil would be ideal) nor as plushy but it does need to be supportive.
Editor's Note: Trish didn't point out which "S" manufacturer is better than the others. But based on her tips, you should be able to figure it out during your shopping."
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up my back. 
