I'd like to hear DH's response to this, but in my mind, if said potential employer feels like you might bail for a low paying postdoc before you've ever met him, you might have a hard time convincing him. If on the other hand, he is open minded about you and your background, it will be a lot easier.What should I do to convince potential employers that I really want to leave Physics, and won't be bailing for a postdoc position?
I'd say tell him the truth. I don't want to teach or live in the desert making bombs, so academia/research as a career is essentially dead to me. I can explain that in an interview. If the interviewer doesn't believe my answers, then there is really nothing I can do about that. If they do believe and understand, you should be fine.
I've found that when dealing with people who also have PhDs and who have also left academia/research for their own reasons, they are more understanding of wanting to leave research. The people I've had issues with are the ones in large companies who employ NO PhDs, where the mindset of many of the employees is 'school is worthless'. Thus, if you spent that long in school, you must be worthless too. It's hard to demonstrate your worth and convince them that you do in fact want to leave research.