"How to buy a mattress - things to consider"

Comfort, Comfort, Comfort: A person spends a third of their life sleeping. All other factors should be secondary to your personal comfort. Comfort styles range from 'extra firm' to 'super plush.' (also known as 'pillow top')

Support, Support, Support: The bed or frame your new mattress is on should have at minimum one leg going straight down in the center of your bed keeping the slat (metal or wood) perfectly level. Additional legs on either side of the center (left to right - under where each person sleeps) are highly recommended. The slat and legs shoud also be centered head to toe.

Warranty: Mattress warranties are void unless purchased as a set. (mattress and foundation) Warranties are only as good as the dealer will back them up. We back up all of our warranties in-house: you always deal directly with us, not the manufacturer, for the full extent of the warranty.; Important information about how to prevent voiding your warranty!

Construction

Foam Encased Edging - a layer of foam aroud the edge of your mattress between the fabric and the springs. Helps keep the edge springs from stretching and poking you when sitting on the edge of the bed after extended use.
Pocketed Coils - each coil is its own wire unnattached to any other and encased in fabric. This helps to reduce transfer of motion between restless and light sleepers. Not as much motion reduction as solid memory foam but better than a normal spring unit. There is no evidence as of yet that this system leads to more sagging.

Fire Retardent Foam - federal law makes this a requirement as of July 1st, 2007.
Brand Name - you could buy whatever brand you like, but if it isn't comfortable to sleep on or the dealer doesn't back up the warranty, you won't be happy with your purchase.
Price - always pick your comfort level first. Then find a mattress of that comfort style in the price range you prefer.