@burcidi-- I second the memory foam idea. You can use a regular memory foam and cut it up as you like. Or they sell mattress toppers especially for sagging that's made of memory foam in stores. A very thin sheet of wood on top of the box spring is also effective. If you have neither, you can place thin pillows underneath the mattress.
The most ideal solution is prevention. Sagging can be prevented by rotating a new mattress every month.