Where are your marketing minutes and dollars spent—on future prospects or current clients? Depending on your answer, you may be wasting your resources.
Focus on the customers you already have—the people who have chosen to spend their hard-earned cash on your product or service. Provide an exceptional product/service and excellent customer service. This will lead to additional sales in the form of repeat business and friends-and-family referrals.
If you prefer more of a hard sell, this post from Heidi Cohen offers multiple ways to encourage existing customers to buy more.