"It is a proven fact that after a time even the most effective sign or banner will become stale. Signs or banners should only be displayed for a maximum of 30 days. After 30 days they must be removed or changed."
This is very simple but very true. The sign is a salesperson that is never off work, never takes a break and always conveys your message exactly as you want it said. Take advantage of that fact! It costs very little money to drastically change the appearance of your dealership, but it does take attention and a plan of action.