Sagging goes all the way down past your mattress’s comfort layer, so not all of it can be reversed. However, you could try using thin pillows underneath a mattress topper to even out a severely saggy mattress. Flipping or rotating your mattress can also help ease sagging. Lastly, you can consider your bed base. Trading your box springs for a rigid bed base or sliding a bunkie board between your mattress and its base can shore up support.