I would second what Frankham stated.

I use the thumb below the frog when playing fast tunes and "rocking the bow" between strings.

When playing airs, waltzes, slow tunes, etc, I use the classical bow hold with the little finger lightly touching the top of the bow.

Then I sometimes change the above procedure by switching the bow hold during a single song, for variety.

The most important thing is "to develop a relaxed, fluid, flexible wrist" (Aerobics for Fiddlers by Carol Ann Wheeler, Mel Bay) She has a check list of things to consider, but I think the most important one is to "change strings by using your wrist, not your arm".

I exercise my hands and wrists before and after fiddling and through out the day whenever I can think of it (like sitting here at the computer). Especially do simple wrist stretching. This is as basic as making circles and figures-of-eight and LIGHTLY bending the wrist of one hand, GENTLY down and back, with the other hand. The same exercises recommended for preventing carpal tunnel syndrome will help prevent problems with your wrists while fiddling.

I am strickly a self-taught amateur fiddler who thinks the fiddle is the most sonorous instrument in the world.