Repetition is especially important in radio advertising. In your case, advertising on the right station at the right time of day might make a big difference. Television advertising is extremely high-impact, but it is expensive. Cable channels are more affordable, but are seen by far fewer people. Yellow Pages advertising is not cheap, but it delivers people who are ready to buy, now.
Outdoor advertising offers high visibility, and the cost per viewer is relatively low.
But outdoor ads, billboards and transit ads, are best used in conjunction with other forms of advertising. All of these media outlets have sales reps who can tell you who reads or watches or listens to that particular outlet. The important thing is to find one whose demographics match that of your customers. Picking an advertising source that delivers your demographic will make all the difference. By finding out when their deadlines are and calling right before that day, you can sometimes buy some unused ad space at a bargain.