It's kinda inevitable, but to help mitigate a sore back you need strengthen and your hamstrings. Also, warming up before hand, just like any other physical activity is key. Otherwise you get blood pooling and will fatigue quicker. Doing exercises like squats and good mornings will help strengthen the majority of your body but, especially the lower back and hamstrings and quads. The thing people forget is that as soon as you bend past verticle/90 degrees you are engaging the lower back which has to support about 2/3 of your body weight. One thing I do is try not to lock my knees and try to support myself against the vehicle(without denting it) with my hips and or knees. Also, you can rest your torso and or chest against the roof rail which isn't usually at risk for denting/flexing. In short, yes you need to strengthen the muscles required to support you will be in these out stretched positions, and not just assume repetitive motions and overuse will do the job. The thing about using any braces or supports is that you end up not engaging the muscles as much therefore they can become weaker which leaves you prone to injury once the support isn't worn. Just don't try to over do it, and listen to your body when it says enough. Plus they add to the risk of damaging the vehicle if you press against it.
Something I thought about was rigging a harness/strap to the ceiling of my garage to help with extended work on horizontal panels.