One, what is the best method for ensuring that your scope is mounted perfectly straight up and down. Second, what is the best device you guys have found for helping to avoid canting while shooting?
Big Sky
B-Square makes a small, out of the way anti-cant device that fits right on a weaver style mount link . I put this on first, then install the scope loosely in the rings. Then hang a piece of string with a weight on the bottom from a tree branch. Next put the gun in a shooting rest, level it up according to the anti-cant device, line the vertical crosshair up with the string and tighten down the scope.
If you don't have a weaver style mount, then my solution to that was a level that fits between the ring screws to level the gun, you'd still have to use a hanging cord like a plumb bub type of thing to get the scope right once the rifle has been leveled. Read more on this RIGHT HERE... I owen one of these and works great.