Loyalty schemes clearly work when they are done on a mass scale — take Nectar and Tesco Clubcard, which promote repeat business, and help the retailers to understand the people shopping in their stores.
On a pub level, it's harder, but success can be achieved if the correct planning is put into place. Think about the offer you are trying to promote and continually question yourself at every stage whether you can afford to give 15% off the next bill, or free desserts on every visit.
Try to capture some data from your customers — a mobile number could allow you to send a simple text to people telling them about your next curry night; an address means you could send out a Christmas menu in November with a special offer to stimulate early bookings.
Basically, the more you know about your customers, the more tailored your offer can be. Remember, if you are asking for some formal registration, you must ask consumers if they mind being contacted. You should not share this information with any other parties without the individual's consent.