Ideally you'll want a mattress that supports your spine equally. This basically means that when you lie on a mattress it'll let your head, shoulders and hips sink in more than the rest of you. This is important since your spine is a double curve.
I've got a king koil perfect contour and absolutely love it. Kinda pricy though, about $700 for a full mattress and box spring.
Also, stretch before going to bed (lightly) and stay well hydrated.
How old is your mattress? Did you turn / flip it so it doesn't develop a hollow spot where you normally lie?