Repetition works Run a series of advertisements week after week or month after month. A series of advertisements repeated often is more likely to get a response than a single advertisement that only appears once. Even if your display is very tiny, in the course of a year you should generate a reasonable response.
But variety is the spice of life Advertisements have a shelf life, just like bread and yours will go stale at some point. People will also begin to 'imitate' it, diluting its power. So have a collection and alternate them.
Track your advertisement's success Some images and headlines work better than others, so keep on eye on the response you are getting and consider redesigning the advertisement rather than dropping it altogether if you don't get much response.
Be prepared Make sure that your support staff know all the details of your advertised product or offer. When a customer responds to the advertisement, anyone who answers the telephone should immediately know what the customer is talking about.