Alan changed the coupon code box label - to "Gift Certificate."
The rationale being "gift certificate" sounds like a paid-for code, making customers feel less inclined to search for codes online in vain.
Newsletter subscribers were sent promotional "gift certificate" orders, so the box would resonate with bona fide promo code holders.
The effect was a 90% drop in coupon usage, with minimal impact on conversion. Says Alan:
"I found that most people were already going to buy before voucher hunting rather than looking for vouchers first. Therefore changing the wording made complete sense. I'm still amazed that other companies don't do it."
His box allows both gift certificate and voucher codes from affiliates, which are still findable in a Google search. Alan has asked sites that post his promo codes to also change their wording to "gift certificate." Not all have made the change, but Alan says customers rarely ask where to enter a voucher code - they will try the gift certificate box.
The reduction in "coupon pinching" is very likely due to the change of labeling.