We spent a lot of money on our mattress, no stains in new like condition except it's sagging! The temporary fix was by buying a 2 inch mattress topper and, depending on how saggy your mattress is before putting your mattress topper on top, fold towels in the sagged area, or a pillow. It varies depending on how minor/serious the sag is!
Our bed is comfortable again, a good temporary fix before spending a lot of money on a new bed!