Try a "memory foam" topper underneath your mattress pad and fitted sheet. Try to get at least one inch thick, but more than 2 or 3 inches is probably overkill.
Also, another quick way to make your bed feel nicer is to get high threadcount sheets. 400 or higher in pure cotton feels really nice. If you're rich, get silk sheets, but then you'll end up wondering how to wash them without ruining them...