Egg Crate Mattress Pad 

A less expensive way to improve the comfort of any bed including sleeper sofas is to add an egg crate mattress pad. They come in many types of foam styles but are typically made of urethane foam.  The topper egg crate style pad will reduce pressure points to help prevent bed sores and can be seen in use in many hospital. It offers cool, dry, comfortable surface that improves circulation by distributing body weight evenly and helps provide a restful sleep experience.  Top quality egg crate mattress pads are made of high-quality medical-grade urethane. These are not the cheap ones you find at Wall-Mart.  The egg crate style provides peaks on the pads to increase comfort.

When purchasing online read the specs carefully.  You may end up with something like this:

"This is not quite the hospital-grade egg crate that I was expecting. It's very thin and doesn't spring back well after being compressed. I needed some extra padding when I was pregnant, so I just used this one on top of the one I already had and it worked well that way."