I leave behind a small orange traffic cone with a sign that says occupied. I usually put that in the driveway area. Typically I also leave a folding chair. Even with those steps, I have had issues. In Rainier I returned for the day to find someone just about finishing erecting their tent in my campsite. They thought I "left". In Canada I had the same. They were desperate and I actually gave them my spot and stayed in parking lot where I could do night photography. I had a similar incident at Mammoth, Yellowstone, where the campground host got confused.
I think there were a couple of other incidents which I forgot or suppressed in my mind. It is a sinking feeling to return to a full campground late in the evening and find your site taken.
