Advice?

I've had maybe 5 out of my 1,500+ sales use the coupon that I include with every order. Right now I offer 20% an order of $15 or more, and in the past I've also offered $5 off $15 and 10% off your entire order. None of these have worked. What kind of coupon would you want in your order, what coupon would entice you to actually return to the shop and use it?