Lift up the mattress to check that the box spring is in good repair. If it's not, flipping it could help or you may need to buy a new one. If the mattress sits directly on slats, move the slats so they're evenly distributed. The mattress may be sagging where the slats are spread too far apart, of because the box spring is broken. More slats may be necessary.
Flip the mattress over and turn it 180 degrees. Repeat this every one or two months to help reduce the current dip and prevent future sagging.
Flip the mattress so the part that sags is underneath. Fill in the dip with blankets or towels. Alternatively, cut up a piece of memory foam to fill in the space. There are also inflatable products sold for this purpose that can be used according to the manufacturer's directions.