- 1). Slide a wide rectangle of plywood between the mattress and the box frame wherever the drooping is most severe. This distributes your weight and it also helps give the mattress more support, preventing it from sagging.
- 2). Place an inflatable air bladder meant to level mattresses between the mattress and the box frame. The bladder is flat when you put it in and then you pump it up while the mattress is on top of it. This allows the air bladder to fill up and support the places where the mattress is thin and drooping.
- 3). Place the mattress on the floor. Unlike a box frame, which only supports the mattress in a specific number of places, the floor will give the mattress full support, removing the droop.
- 4). Cut a piece of 4-inch memory foam that is roughly the size of the drooping portion of the mattress, using an electric carving knife. Slide the memory foam between the mattress and the box spring. This helps support the mattress by pushing the drooping part back up.
Source...