Face reality—an advertising sales rep is not exactly a power broker. At best, you may be wearing a power tie. You can't guarantee a client's ad will appear in a certain position in a newspaper or magazine. You can't tell him he'll generate enough sales from the ad to justify its cost. Clients are not stupid. They, too, know you don't have that power.
However, you are absolutely brimming with another kind of power: You alone hold the key to an ad sales rep/client experience that enables both parties to win. You conduct yourself with professionalism, prepare well for your client interactions, and provide excellent ongoing service to advertisers large and small. Stick to these principles, and you have the power to control your advertising sales career. You also have the power to flush it directly down the toilet. We recommend you don't exercise that one though.