If you go to the Customer Edit page for any Shopper, you'll see a PROMOTION area on the far right.
Selecting Preferred Customer from the drop-down and assigning a Discount percentage will automatically give this customer an additional discount when they buy anything online from your site. This discount would be in addition to any Coupons they have, but it will take into account the Sale Price. Meaning, if the Preferred Customer's discount price is less than the Sale Price, it will use that instead and vice-versa.
Some of our clients have been extremely successful in taking their top 100 customers, let's say, and sending them a very nice letter in the mail stating they are one of your Preferred Customers, and whenever they shop online with you, they'll get an additional 20% off (or whatever you want to give).
What's nice about this approach is the consumer doesn't need to do anything different! They simply place their order, and the additional discount will automatically be applied at Checkout.