Testing, Testing, 123...

 If you go to the grocery with $5 in your pocket, there is only so much food you can take home before you see flashing lights in the rear view mirror. Not so with marketing $$$. You could spend $10,000 on a campaign that gives you 5 customers or it could give you 500 customers. The difference is in the message. Changing a single headline can change your response rate significantly. Same with the wording of the offer. So always test out different versions of your marketing pieces before rolling out a full blown campaign.
One of the biggest mistakes in marketing is not testing.