Hi Jesse -
First, welcome to the site.
Second, glad you tried this in a parking lot and not on the road! Seriously. I bet there was some adrenaline.
I made a kill switch for my car, though I also use the technique in other cars without one. I can't say that I've ever mistakenly turned to "OFF" and locked the steering, but like most driving tasks it'll probably take a lot of repetition before the correct way to do it becomes a habit.
Aside from the kill switch, I can't think of any other easy "fix" for the problem other than careful repetition.
And I wouldn't recommend using the technique if you're not 100% comfortable with it and understand the potential risks. (It sounds like you're aware
)