My prefered method is to mark a soapstone line with a speed square and use a C clamp to clamp the bar aligned with your mark, though I'm using some vise grips here. I've also found a heavy application of soapstone to the bar greatly helps the side of the tip glide smoothly along the edge of the bar.
While it takes a little time to set up, It can very drastically reduce the amount of grinding needed after cutting. Below are a piece of 3/8" plate and two pieces of a 1/2" thick grader blade which given the much higher carbon content does not torch cut nearly as well as mild steel does. Given that it doesn't grind so easily either a clean cut is very important.
With a 1/8 welding rod for reference.
It won't remove the human element entirely or the need to cut with a clean properly sized tip and the right pressure, you can see certain issues with my cuts from a combination of human error and not worrying about torch settings, but it's still a major help.
