This is the simplest of the 5 steps and it can be easily done at home. This step is about making up your mind about different materials that seem to give you the same feel, comfort, and support, based on their durability. Different mattress materials have a wide difference in durability and in how long they keep their desirable properties. It is unfortunate that too often a mattress that was perfect in the store and for the first few week when you took it home felt wonderful only to find that this feeling didn't last very long. Some of the materials used in mattresses these days will only keep their qualities for a few weeks or months and completely break down not long after. Some of these materials are even found in mattresses that are very expensive.
In general, it is important to know that materials used in a comfort layer will generally be compressed more deeply and more often and so will soften, compress, or break down sooner than materials used in the support layers of your mattress. This makes it very important to choose the highest quality materials that fit inside your budget in the comfort layers especially as your mattress will only last as long as its weakest link and this is almost always the upper comfort layers. At the other end of the scale, there are many types of materials that will last for many years (8 - 12 years is a reasonable expectation for a quality mattress) and many materials will last well in excess of 20 years. While there are many factors involved in the durability of different materials ... this is an approximate or general rating of how long different materials may last when used in the different layers in a mattress so you have a rough comparison of the relative durability you can expect. In the basic guide to pricing, you will see the approximate budget ranges that you can reasonably expect to find different materials and here we will list them in the approximate order of durability. Bear in mind that how long a material lasts will also depend on the type of use (and of course sometimes "abuse") it is exposed to and on how well you maintain your mattress by either turning it or flipping it if it is two sided (which will last much longer than one sided mattresses).
In the comfort layers from longest lasting to shortest expected lifespan (keeping in mind that these are generic guidelines only and are not exact because there are many factors involved in the relative durability of each material category and there is overlap between categories depending on the specifics).
Latex foam, Natural fibers properly constructed, and solid gel materials.
Microcoils (an estimate as this is not known for certain)
High quality memory foam (5.0 lbs density and over made by a known and certified manufacturer)
1.5 lb HD polyfoam or higher (I would use 1.8 lb as a guideline in a one sided mattresses)
Mid quality memory foam (in the 4 lb density range made by a known and certified manufacturer)
Conventional polyfoam less than 1.5 lbs (not usually suitable for use in mattress comfort layers because of durability issues except for occasional use, very low budget ranges, or in thin layers that total "about an inch or so" or less ... usually in a quilting layer)
Cheap memory foam in the 3 lb range or less (same comments as conventional polyfoam)
In the support layers from longest lasting to shortest expected lifetime (again these are generic and different categories would have some overlap). Keep in mind that the support layers of a mattress are not generally the weak link of the mattress.
High quality tempered innersprings
Cheap lower quality innersprings.
Conventional polyfoam (not suitable for use in a mattress core except for occasional use because of poor resilience and other qualities and because it won't last very long)
Waterbed support cores that are well made and that don't develop leaks or other issues can be expected to be in the upper range of this list
Airbed support cores that are well made with the best materials and that don't develop other "issues" can be expected to be in the upper range of this list.
One final note about durability and that is to never trust a mattress warranty which are not a reliable indicator of the useful life of a mattress. They all have exclusions that are designed to protect the manufacturer from honoring them or exclude "normal" body impressions and don't cover the loss of comfort and support that comes from foam softening. In a mattress that has natural fibers in the comfort layers, some degree of body impressions are a normal part of the breaking in process and don't affect the comfort or support of the mattress. They are not indicative of premature wear or the softening or breakdown of the material. Natural fibers will compress and become firmer but not wear out for a very long time. In a mattress that includes polyfoam, memory foam, or other synthetic materials however, body impressions are a sign of wear and the breakdown of the material and should in no way be considered "normal" even though a warranty will not cover this unless they are deeper than the warranty exclusion.
Long before a mattress develops impressions that remain without any weight being applied (complete breakdown of the material) it will become soft and fail to perform to its original specs even though it may just have enough resilience left so that unweighted impressions are less than the warranty exclusions. Warranties for most mattresses that include polyfoam in the comfort layers are meaningless and meant to be a sales aid or closing tool for the sales outlet to help them sell certain mattresses at a higher profit much more than they are consumer protection against the "normal" weaknesses of inexpensive materials. Beware of any sales person who attempts to sell you a mattress based on its warranty and bear in mind that some of the highest quality and longest lasting mattresses in the world only have a 10 year warranty. In other words, base your durability estimates on what you know about the materials in a mattress, not on manufacturer warranties ... particularly in the case of polyfoam or memory foam.