Years ago I wrote this on my blog:
Saddles are a very personal fit item. If you are like me this is probably the bicyclist’s equivalent of the search for the Holy Grail. To me the important part is getting the seat cushion part to line up with my sit-bones. How do you do this? My thought is to take a piece of rather soft styrofoam about 3/4″ thick as a template to capture your sit-bone size, position and width. Here is how to do it:
* place the foam on a hard surface where you can sit as you would on your bike
* sit on the foam, being sure to make the sit-bones produce an impression into the foam
* now you can get fancy if you like, make a plaster-of-Paris casting of the impression
* take this casting or sheet of foam with you when inspecting saddles at your bike shop to compare your position versus the saddle positions.