Use a firm mattress topper
Place a plywood board under the mattress
Rotate or flip the mattress if it is designed to be flipped
Tighten or replace the bed frame and slats
Add more support slats to the bed base
Remove plush mattress pads or thick toppers
Use a firmer foundation or box spring
Check for sagging and replace worn support parts
Let the mattress fully expand if it is new
Adjust the room temperature if the mattress softens in heat
Replace the mattress if it is permanently worn out