Every so often when the mood strikes me (IE I have money) I head down to the local hardware store and pick up a handful of professional metal sticky signs. I like push/pull (for the opposite door orientation of course), 'employees only' (for any door I can find) and 'please use other door' (I stuck this one onto the inside side of a bathroom door at a Subway with no other door and it stayed up for over a month) the best, but I'm sure any hardware store selection will keep you busy for a while. They're awesome because I see them up legitimately everywhere so when people see them they assume they must be there for a reason.