Came up with this last night at the range. Someone else probably thought of it, but what the hell.
Take an alignment rod (not one of your clubs, unless you don't like it very much) and line it up directly behind the ball along the target line. How far from the ball depends on how decent of a ball striker. I started out with the rod about 6" behind the ball, and after 5 balls or so moved it up to 2" -3". Once it's this close you can basically guaranty that if you don't hit the rod, you're hitting it pure. Having the rod there also helps you groove a good takeaway.
There's a lot of talk about mats masking heavy shots, and this sort of fixes that problem. Don't get me wrong, grass is still preferable for a lot of reasons, but sometimes you can't get to a grass range.
Last thing, this really only works with a 6i on up. The shallow AoA of a long iron means you can make a great swing and still hit the rod.