Anyone have any suggestions for securely clamping round stock to prevent it
from turning while I'm trying to thread the end of it?

I've got 3/8" mild steel rod that I'm threading with a 3/8-16 die. I'm
clamping it in a smooth faced 6" vice and theading it there, but the rod
turns on occasion marring its finished surface. Any tips on claming it more
securely to prevent it from turning while threading?

Thanks