Ever since we created Postcard Promotions in 1991, we have produced effective direct marketing pieces for 100's of organizations and we've seen what works and what doesn't. For example, you could feature a web site or provide an option to request a free information package. Magic words such as FREE, New, Guaranteed grab attention.
Deadlines create action.
Formatting Your post card needs to pass the quick scan test. Have someone not familiar with your campaign review the card and see if they can identify what the benefits and offer are within 10 seconds. Your prospects will are more brutal - they'll give you around 2 seconds to either grab their attention or fail this test and end up in the trash. The more you try to add to the message text, the less you will ultimately convey. Avoid the cluttered look at all costs.