Buyers often get caught up in the feeling of a soft mattress under them at the furniture store, only to realise it leaves them with an aching back after purchase. The good news is that there is a simple solution to add firmness to your soft mattress.
Visit your local home improvement store. Ask a store assistant for a cardboard refrigerator box. Request two boxes if you want a really firm mattress.
Lift mattress off of box springs. Lean it up against a wall for support.
Flatten out a cardboard refrigerator box. Lay cardboard on top of box springs. Use a utility knife to cut off excess cardboard. Fold or cut box in half if you are only adding firmness to one side of the bed.
Repeat Step 3 if you are using two pieces of cardboard for additional firmness.
Place mattress on top of box springs and cardboard. Make the bed as usual.
Use new cardboard pieces every two weeks to renew firmness. For maximum firmness, use a plywood board underneath your mattress instead of cardboard.