There is lots of information out there about saddle fitting, some good, some not so good. Here are some very useful guides and tools for understanding how to fit a saddle correctly
The passier saddle fitting guide is a very helpful short paper on how to measure and fit a saddle.
For a more detailed video on the more technical and complex aspects of fitting a saddle the following video of Saddle fit may help.
How to choose the appropriate gullet for your horse is detailed in the video below.
(Here is the link as well in case it doesn't work http://youtu.be/U2mKz0uP_K8 )