Memory foam isn't as new and mysterious as it was in the late 1990s and early 2000s, but there are still huge differences in the quality of foams—and not knowing them will sour your experience in a hurry.
What separates memory foam from other popular bedding materials is its ability to form to and support your body while you sleep
This is made possible by two factors: visco-elasticity and density.
This is a term that comes from the words viscous and elastic.
Viscosity is a term mostly used of liquids and it's best illustrated as the difference in hot and cold molasses.
Hot molasses is much thinner and will pour quickly while cold molasses is thicker and pours slowly.
Memory foam is much softer when it's heated and firmer when cold.
This is what allows memory foam to fine-tune itself to your body when body heat is absorbed into the foam.
It will soften beneath your body allowing you to sink into it
The elastic quality of the foam is its ability to return to its original shape after being stretched or compressed—just like a rubber band.
The most important consideration when you buy a memory foam mattress is the density of the foam.
The density of a higher-quality memory foam is 5 lbs. or more for a mattress, and 4 lbs. or more for a memory foam topper.
To get the optimum support your body needs with a mattress, you should look for 5 lb. density because it lasts much longer than the less dense mattresses.
You can check the support that a mattress provides by lying flat on your back (don't raise up while doing this) and sliding your hand under the small of your back.
The harder it is to slide your hand in between the mattress and the small of your back, the better the support.
A 4 lb. density mattress is good if you are price-conscious and will normally be considered a 10-year mattress. This is a great way to try out memory foam without breaking the bank.
You should also insist on an American-made foam. Watch for trick wording like "Made in America" because many who make this claim are using Chinese foam and are only doing a little of the manufacturing in America, so they can claim it's American-made.
A couple other hints you might want to keep in mind is that many manufacturers will call the mattress a king, queen or other standard name while manufacturing it to smaller dimensions. Be sure to check the actual dimensions of the mattress so you're getting a standard size.
Foam is subject to shrinking in cold and expanding in heat so also allow an inch or so when you measure.