Check your local laws or store for Purple Posting Paint. I'd venture to guess most states have adopted the purple paint as a "No Trespassing" warning. The purple paint has some fairly specific requirements, like painting trees or fence posts every 100' and certain distances off the ground...
I would also not allow some game warden or deputy try claiming a sign is needed at all. A person knows if they are not on their own property, and don't have permission, they are trespassing....no signage needed!
