This tip is a such a slam dunk that if you don't follow up on it I don't think you can seriously say you value or work on relationship marketing: Find out the birthdays of your clients, contacts, referral sources, and other important people's (like how about family and friends, too?)and do something to recognize them.
In my experience the best way to say "happy birthday" is with a greeting card - a real card, not an e-card. You can also call, or send a note on Facebook, or a note via e-mail, and all of those are fine and generally appreciated. Nothing, however, has the power of a real birthday card.
This technique isn't new and it isn't a secret. Top sales people in many fields have (insurance, car sales, and real estate come to mind quickly) have written books and give their own seminars about the power of sending cards in relationship marketing. You probably know restaurants or retail stores that have "birthday clubs". There are no end of different ways to recognize someone's birthday and it's a key strategy.
When you send a birthday card, or use some other means of recognizing a birthday, the less you make the card about you and your business and the more you make it about the recipient, the more powerful it will be and the more appreciated. A sincere, simple, Happy Birthday message in a card is a lot more effective than a card in which you enclose your business card or talk at all about your business or service.
The more you you mean it, the more it matters.
