If you try to sleep on a soft or sagging mattress, your spine may not be supported properly. Improper support of your body while sleeping causes pain and even an inability to sleep. If you're not in the market to replace the mattress, or if the mattress is not yours, you can effectively firm it up with one of two methods. What will work for one mattress may not work for another, so be prepared for trial and error.
Measure the length and width of your mattress and write down the dimensions for future reference.
Purchase a 4- by 8-foot sheet of 3/4-inch plywood and cut it to the dimensions of your mattress with a circular saw. Alternatively, give the dimensions to an associate at your local home improvement or hardware store where you bought the plywood and request that he cut it for you.
Slide the sheet of cut plywood underneath the mattress and on top of the box spring to give the mattress more support.
Purchase a firm mattress pad that fits the size of your mattress -- twin, full, queen or king.
Unroll the mattress pad from the packaging and place it directly onto the bare mattress.
Place a fitted sheet over the firm mattress pad to hold it in place on the mattress. Make up the rest of the bed.
If you're unsure of what type of mattress pad to get, ask a sales associate to assist you.
The home improvement or hardware store may assess a fee for cutting the plywood. Ask in advance, so you know what to expect.