I found another interesting link on saddle fitting when looking for a Silver Royal saddle, like the one Cattypex had mentioned. This one offers a couple of different ways to look at and check for good saddle fit.
Four Winds Saddle Tack Store in Fort Collins Colorado has a page online listed as Withers Tracing and another one of Saddle Fitting. I realize a lot of the information is based on or shows english saddles- all purpose, close contact and a lot of dressage saddles. I haven't found one yet showing anything more for western, so I will either have to keep digging or get some pictures taken of the boys out back.
One of the main things that I find interesting in the links here for wither tracing is the flexible curve available at the office supply store. I may be a bit brain dead on this one, but what would an office use those for? At least they are useful in helping you establish a drawing of how your horses back is shaped, but like Kitt had posted the pictures on the saddle fitting blog, with time and exercise or time off, the shape will change and saddle fitting requirements will too. Muscles change as the horse is brought back into shape, change more as you continue to train and develop different things and deteriorate when the horse has had an extended lay up following an illness or injury.
Saddles are an investment like a lot of other things. But spending money on a nice saddle and finding out it doesn't fit you, your horse or both is a bit disheartening to say the least. But if it doesn't fit, you can always resell it and recover some of the money to put towards one that does.