I read in a marketing book that simple and catchy works best. For an online store, the cleaner the better. For mine, the background of the card is the same as the banner of my shop, for consistency; then I have my shop name, my slogan, and my site on the front, while on the back I have my shop again, in larger font, just for good measure, along with my blog. When I hand them out, I handwrite a coupon code on the back for a personal touch. :-)