There is nothing worse than poor nights of sleep from a sagging mattress. Though the best solution is to buy a new firm mattress, it is possible to wake up without an aching back if you fix your current mattress. The following can help save your back and your wallet.
Lift the mattress up and assess the cause of the sagging. If your mattress is supported by a firm box spring frame, then it is most likely the actual mattress and springs which are shot, and you will need several blankets for the remedy. If your mattress is resting on a flexible frame and this visibly bends when weight is put on the mattress, then an extra brace is needed.
Understand that if it is the mattress itself that sags, you should find a few extra blankets lying around the house and fold them into rectangles approximately 2 by 4 feet. Fleece works well. Depending on how large your blankets are you may only need one or you may need up to three, but you want them to be about 2 inches thick when folded. These will be used to support the sagging area which usually forms at the centre of the mattress. You will need to shift them around or add blankets until the mattress becomes level as desired.
If it is the frame causing the sagging, then measure the width of the frame the mattress fits into. Check what extra wood might be in your garage. A piece of plywood cut to this width and at least 4 feet long will work. Several boards of at least ½ inch by 4 inches cut to fit across the frame will also work. If using boards, any pine or spruce will do, use at least three of them spaced 8 inches apart. The centre board should be in the exact centre of where the mattress sags. If you have an extra wide mattress, such as a king mattress, then extra support may be needed. Make sure that the boards you are using are sturdy. Hardwoods such as oak will be best. Placing breeze blocks or something of equal sturdiness beneath the centre of the middle board will add even more support.
Know that in addition to creating support beneath where the mattress used to sag, a top layer of 2-inch memory foam in between the top of the mattress and the fitted sheet creates extra support and comfort. This is the icing on the cake. It's not a new mattress, but it will feel a whole lot better and doesn't cost as much as buying a new one.
If you need to fix the support on the frame beneath your sagging mattress and need to cut boards to fit across the frame, a powertool is not needed if you don't have one or are unfamiliar with using one. A standard handsaw works fine.
Extra caution is advised when lifting mattresses and using a handsaw.