1) I do a business card swap with three other Etsy shop's we send each other business card's or some kind of coupon. What I do Is a small 1/2 the size of a business card with either 10% off or free shipping I make sure I have a code on them so I can track them.
2) I add a special thank you card. These are pre printed card's that basically say. Thank you for shopping with me. Please enjoy 10% off your next purchase and I have my shop name on them and address I don't really have to add a code to these because I can track where they are coming from.