An age-old trick for making a mattress firmer is by placing a sheet of plywood the size of the mattress beneath the mattress to add stability and help prevent sagging. You can also try flipping your mattress, replacing a box spring that is worn out, adding a firm memory foam mattress topper, using for a wood slat bed frame, or placing your mattress on a solid platform bed frame, or even on the floor if that appeals to you. But if your mattress is both sagging and past its prime, keep in mind that mattresses are not meant to last forever, and it may be time to replace it.