Purchase a mattress "bladder," which is an inflatable bubble that resembles a large, flat pillow. Pump up the bladder according to the manufacturer's instructions. The bladder pushes up against the sag, raising the valley to the mattress's level. The bladders are available in home goods stores.
Buy a piece of memory foam -- available at department stores -- that's slightly larger than the size of the sag. Cut a piece of memory foam so it slightly overlaps the sagging area. Put the memory foam between the mattress and box spring to raise the sag.
Flip the mattress over so the valley of the sag is now on top of the mattress. Fold two to three thin blankets in the approximate shape of the sag, and position them between the box spring and mattress. Test the bed for comfort, and refold or add more blankets as necessary until the sagging area is level.
Inspect the box spring for signs of distress. What feels like a saggy mattress could be a box-spring problem. Add more supports, if necessary, or consider replacing a worn out or broken box spring.