Measure the dimensions of your bed and write them down. Purchase a sheet of plywood that matches these dimensions. Place the plywood on top of your box spring. This eliminates any general sagginess that may have resulted from the creasing of the mattress.
Flip the mattress so the underside of the crease touches the plywood. Turn the mattress so that the bent portion is closer to your feet and legs and away from your back and neck.
Place flat disc weights, cumulatively weighing up to 100 lbs. directly onto the convex crease. Leave these on for 24 to 48 hours. Check the crease after the designated time. If it's not significantly reduced, leave it on for another 24 to 48 hours.