The edges and sides of a mattress are potential weak points that tend to give way first. Many of the better more comfortable and durable mattresses have reinforced sides that resist sagging and maintain their strength as they age. The effective life of a mattress can be extended by regularly turning it end for end and also reversing the sides. This tends to even out the deformation of the springs with age and use.
The cover of the mattress also matters! A good cover should enable the mattress to breathe and dry out rapidly - everybody perspires to some extent when sleeping, and it is important that the cover allows a good degree of ventilation. A natural bamboo fiber acts as a wick, pulling perspiration away from your skin and enabling it to easily evaporate. Bamboo fiber can absorb three times its own weight of moisture, but also enables your bed and mattress to breathe offering a much more comfortable sleep than without it.