It really is tricky. My dad wouldn't dare get behind the wheel now, but my stepmom still drives (short trips) and for god-knows-what-reason they bought a brand new car a year and a half ago. And they were going round and round about moving, when my dad, out of the blue, decided to move to assisted living (my stepmom wanted to do it a year before). I think he could see how much stress she was under.
We were very lucky, most of it sort of took care of itself. With my dad, my brother and I gave him big props for doing the SMART thing, and my dad just beamed. We are fortunate that positive reinforcement works pretty well with him.