I use them for repeat customers, so very useful. I enclose a coupon code for future purchases. I also personalize the coupon code with their name so it's only for them. My way of saying thanks for returning to my shop.
I do the same thing that Alex does. Each purchase comes with a loyal customers coupon code. Many of my customers are repeat shoppers, so I'd say it does work a little at least.
I've tried a few, Black Friday/Cyber Monday, SuperBowl Sunday, Even one just for my Birthday. I only ever got sales from the birthday one, but I'm thinking that's because i had Franklin dressed up in a party hat and people couldn't resist...
I offer promotional coupons time from time in my shop. It does takr a little time for buyers to notice it but they eventually do.
I don't think any of my coupons did any good job if I offered them over the weekend - the time is too short for the offer to get noticed. However, instead of marking each listing down when I want to do little sale or promotion, time from time I offer a discount coupon for a limited time of 4-6 weeks. And this offer does work well and 90% of buyers within the period of offer get their savings. Besides, the sales increase as well as the amount of new customers.