Hold your release hand naturally. There are some great shooters that hold their hand vertically or turned even more. Jeff Hopkins turns his so much that it looks painful to me, but he gets awesome results.

Larry Wise teaches that for most of us a natural position will be better since it is easier for you to relax your arm and hand. I had been shooting with my hand almost vertical with the thumb down, but when I tried it with the natural position, my scores went up almost 10%. I made a couple of other changes at the same time, so I can't give all the credit to hand position, but it definitely helped me.

Hope this helps,
Allen