This isn't mine, it's something I learned from Al Carruth, it's the most amazingly clever thing I have ever seen.
The story goes that Al came up with this technique after he broke his arching plane.
To arch his braces, Al takes his Bailey no 4 and super glues a tiny brass shim about 4 inches or so back from the blade. He has two lines that he marked across his plane, one for back braces, one for top braces, with the shim glued in place at each line he gets the arch he's looking for. The super glue holds the shim securely enough for planing but it's easy enough to take off.
This photo will give you an idea of what the plane with the shim looks like:
Then he clamps the braces in a vise and planes away. I'm showing just one brace here but he does all four back braces at the same time, and it works better if you do that.
And out comes 4 braces with the correct radius on them, believe it or not!
For anyone that wants to try this at home, here's a web site that will help you with the math to figure out the size and location of shim you need for a given radius.
http://mintaka.sdsu.edu/GF/explain/atmo ... rizon.html
I stopped using this because for me the hardest part is not getting a decent arch, it's keeping the plane perpendicular to the side of the brace. So I just draw an arch on the brace and then use a block plane on a shooting board and plane to the line.
Al's technique with a shooting board ought to give a perfect arch every time. I'm going to have to try that.
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I like simple solutions.