This is the one issue I think troubles me the most. I don't think my DS is particularly steep, but I know that I do roundhouse somewhat. I guess I thought I had this issue licked but a couple holes into a round it comes back.
I find that in practice (indoors) I can keep it in check pretty well by striving for pitch elbow condition. On course however, that roundhouse move creeps back in. Again, in practice the focus is in the feel vs target so its easier.
I have viewed my swing on tape plenty, and a small but noticeable shoulder swerve starts my downswing, and pulls are the result. I'm sure its more noticeable on the course. My swing used to be all shoulder motion during takeaway and downswing, its tough to shed those old habits.