Some sophisticated bed cushioning products include an air or water pump which maintains pressure throughout the pad, or alters it gradually, alternating pressure changes across the body to provide relief to one part while the other is more supported. Even though my mattress is extra soft, it still needs something to help spread the pressure more evenly. An egg-crate pad has worked perfectly. I rotate it every so often, and at some point have to get a new one because it gets sort of compressed. I also like it because when my girlfriend spends the night, it's comfortable for her too, and as it's under the sheet, it doesn't make my bed seem like a hospital bed.
Sheepskins are also popular bed cushioning.
The wool is trimmed and the sheepskins laid under the bottom sheet. They are soft, comfortable, and minimize pressure points, particularly on the hips, elbows, and knee joints which tend to get the most contact while laying down. Sheepskins tanned using glutaraldehyde-chrome hold up better in the wash.