Frequent Buyer Program
If you think about it, the existing customers are already sold on you, your business and your products or services. You originally invested time and effort to sell them, and you do not want to waste that investment. Hopefully your products or services combined with your post sales customer service have ensured that you still have a degree of goodwill with you customers.
Goodwill is the friendly feelings and positive attitude your customer has towards you and your business. Unfortunately that goodwill will depreciate over time, unless you do something to renew it. That could be a point of contact or even better a further sale to the customer.
Points of Contact
A point of contact is when you connect in some way with the customer, such as a follow up call or a newsletter, or when you deliver further products or services to the customer. It gets you back in the customers field of vision, so they are aware you exist and you are working for their best interests.
It is best to plan out a schedule of these points of contact, based on your past experience of what works well. For example, in a lot of restaurants, a short while after the food is delivered to the table, the waitress is required to ask how the customer likes the food. It gives the restaurant a chance to salvage any issues with the food, and shows to the customer that they care about the quality of their dining experience.
Other schedules can be based on an annual schedule, such as spring sales, fall sales, summer clear out sales etc. Be sure to keep it below the pain threshold and include an opt-out mechanism to avoid becoming spam. You want your customers looking forward to the points of contact, the way some folks look for the new Ikea or Sears catalogs.
Get Their Contact Information
Back to the frequent buyer program, so you make the first sale and create a customer. Be sure to get their contact information, which allows you to keep in touch. Often this is done as part of the sale. In retail, you may have to apply a little finesse. Then schedule the points of contact. In your communication you need to offer them a reason why they should come back to you rather than your competition. Remember that the biggest thing that pushes people away is the lack of interest in their business. You need to show you care, and offer something special for your "select customers".
These days, with computers it is easier to tailor each point of contact to match the interests and past purchases of your customers. Rather than a blanket approach, you can offer specials that are more likely to fit your customer's needs, with improved results. It shows you value your customer's time by not wasting it with irrelevant promotions.
What You Reward
Think about what you want from your customer and how you can reward it. For example, a restaurant I like has a business card they stamp for each meal, and on the 10th meal I get my meal free. If your program is not working, perhaps you are rewarding the wrong behavior, or offering the the wrong incentive. In that case, it pays to test different things to find the right mix. Don't be afraid to ask your customers what they think would work. Since you are not trying to trick your customers, you are in effect entering into a contract with them.