I bought a mattress back in June. It was nice and firm back then but with a pillow top for comfort. It really helped my back quite a bit. I can't remember the type but it a bigger/better name brand. Right now it is saggier than the 15 year old mattress it replaced and I am constantly sore. Short of sleeping on the floor, putting a piece of plywood down on top, and/or getting a new mattress, is there anything out there that will work to make it firmer? Something like a mattress liner than goes on top? If I have to get a new one, I will but I won't even be living here after a year to eighteen months and would prefer a cheaper alternative if at all possible.