Radio is a background medium and listening behavior is pretty much set. people listen to the same radio stations at specific times day in and day out. This, one spot is listened to 6 or 7 times a week by the person who listens to the radio station at the time when your ad is placed. If he listens another time, he will never hear your commercial. That is why advertisers on radio pretty much spread their spots in various dayparts.
Radio is effective when the material is created to harness the audience's theatre of the mind. If the listener can see products or visuals when he/she hears the ad. Otherwise, the ad is so immemorable and inconsequential, it could even be a nuisance.
Radio is a passive medium. The listener will not stop what he/she is doing to listen to every word you say in your ad. Work around this limitation. Thus the ad should be entertaining while intrusive. And should be tested for its noting score or the ability to be "heard/noted" and remembered.The best way to find those who know how to do advertising on the radio http://www.whereorg.com/Radio-Advertising-Inc-18678670