These are great! Here are a couple I ran into over the years that stuck with me:
Seen in a motorcycle shop, posted over expensive helmets -- "If you got a 10 dollar head, buy yourself a 10 dollar helmet."
Seen in an auto repair shop -- "shop cost per hour: 30 dollars; if you watch, 35 dollars; if you help, 40 dollars.
The most common trespassing sign seen in America usually says something like this: "Trespassing Forbidden, Violators Will Be Prosecuted" While out for a motorcycle ride in a remote area I encountered this version: Trespassing Forbidden, Violators will be Prosecuted and Prosecutors will be Violated. (Apparently, he not only didn't like trespassers he disliked law enforcement even more.)
Saw this one in a gun store in Kansas City: "All bits and pieces of anyone caught shoplifting will be prosecuted to the fullest extent of the law." (Well, it deterred me!)