There are several factors at play to keep your customer base loyal to you and your services. The first and foremost is to put yourself in their shoes-you're a business owner who wants to grow his business through advertising and marketing that is reasonable and effective.

1) Treat people the way you wish to be treated-this mantra has served me well in my work life as well as my personal life.

2) Let the owner know you genuinely care about their needs, situation and aspirations and make sure you sell them only what they need. Because they have other media they may need to budget for and since you do not offer that media-guide them where their money is best suited even if those dollars are not all going to ward your product. 
As a marketing professional I do not initially go see a client to sell them something-I show them our capabilities and they decide whether they wish to purchase our television and or sophisticated digital solutions.

3) Listen to them-to their concerns, their needs, what has worked and what has not and make sure having listened you are able to put something into action a plan that is helpful and will will work.

4) Make sure your client is comfortable with you and your product at the onset and through the campaign. 

Having been an advertising and marketing professional for over 15 years, I attribute my success to aligning myself with company's that are relevant, have a great reputation and innovative problem solving solutions. Additionally, doing what it takes to make the customer satisfied in every way.