From having built a few of these, my solution was to add a string to the end of the URL's in the promo material (ads, email, whatever) that, when clicked on, wo

From having built a few of these, my solution was to add a string to the end of the URL's in the promo material (ads, email, whatever) that, when clicked on, would trigger the coupon code box on the site for that user only. Otherwise you didn't see it. The company I worked at that used this did a lot of targeted and 1-off promotions so it seemed to work pretty well.