In South Carolina a business that wants to prohibit CWP holders from bringing their firearms must post a very specific sign. The law mandates the size, color, wordage and graphics as well as proper placement. The upshot of this is that I have yet to see a legally posted place of business. My CWP instructor said that he has seen one sign that was actually legal but even then it could be ignored because not all public entrances were posted.
Another nice thing about the SC law is that the minimum size of the sign is larger than 8 1/2" by 11". This makes it more difficult for some business owner to just slap a sign up after an incident.