Alot of it is simply common courtesy and common sense, unfortunately not so common.

Most will do the right thing, they will move on quietly if they come across you, some will be jerks and move right in. On public land, in general, you have no "reserved" spots, just cause you hang a tree stand doesn't mean its your spot. If you come to the one spot you scouted for weeks and someone is already setup in there (though not in your stand), then you should just move on. However, politeness also says if he saw your stand he should have just moved on as well.

It does happen that your stand won't be noticed. I did it once, headed into a spot I had found, climbed up a tree with my stand was all settled in and "wow where'd that stand come from?" Never saw it until I was in my tree, it was 15 yards from me. I stayed, but if the other hunter had come along, I would have gladly left, hadI noticed the stand before I was setup, I would have also moved along.