Postcard Marketing has been used to generate sales for all kinds of businesses. Nextmark.com is one of the places where you can go to rent a mailing list from a long list of categories.
The reason why postcard marketing can be so effective is because you can send postcards to people that you know have a specific interest in something. All you need is a good postcard to send your message, and a good list of people to send your message to.
After testing and tracking your results, you can continue to send out postcards for as long as your postcards prove to be effective at bringing in revenue.