This is the way to go if your saddle is too saggy. Some leather saddles already anticipate this by providing the holes in the wings and a cord to tension these such as these ones.

http://www.spacycles.co.uk/products....=m2b0s204p2667

It's worth noting that the saddles I've linked have much thicker leather than the Brooks B17 and longer rails and work on the "hammock" principle. They are already waterproofed which is a help also. These saddles appear to be marketed under the VeloOrange brand but at a much higher price. In my opinion, and I've used Brooks saddles for over forty years, they are a superior product.