Learn how to get perfect posture in your car by adjusting your rearview mirror; featuring Jackson Sayers, BSc (Kinesiology). One of the easiest things you can do in your car for perfect posture involves your rearview mirror. The simplest thing to do is just sit up as straight as you humanly can, and get the mirror going to the maximum height that you can. The nice thing about that is that every time you get back in that car, the rearview mirror is already set at the proper height for perfect posture. It's going to force you to get back in your seat, sit up really high, and see what's coming up behind you.