Our mattress is about 10 years old and my DH and I both have back issues. I need a soft mattress- he needs concrete hard. I bought a twin size memory foam pad for my side of the king bed and I love it! It's only a couple inches thick, but provides me with just enough support so I don't hurt. It's only minorly weird that I sleep a little higher than him at night.
We really want a sleep number bed (in-laws have one and love it), but we just can't seem to shell out the few thousand for it. Stupid, because we'll both shell out thousands for new bikes without blinking an eye- but for a good night's sleep we just can't justify spending the money. I know only you ladies could understand that.

Try the thin pad and see what you think.