The trouble with a majority of promotional coupon campaigns (especially during the holidays) is that their intrinsic value is often much higher than their perceived value. That is to say, they cost more than they are valued by the recipient.
Here's a big idea. Offer a coupon card whose value has a minimum (say, $15) and a maximum (say, $500). The actual value of the card can only be determined at checkout. Then, send the cards to your loyal customer base, ideally multiple times.
Imagine, if you will, the excitement of purchasing something knowing it's $15-off but hoping it's up to $500-off. That beats a static coupon any day!