This is critically important. Especially if you're in a business that typically doesn't keep their customer names and addresses because you're missing an incredible amount of money. I cannot stress this enough. I know this sounds silly, but it's more difficult than you think. Many consultants will tell you to save money by using 'bulk-mail' postage or as its now called 'standard rate'.
Bulk rate is the dumpster rate.
After taking the proper steps to get the best list, don't blow it by being cheap on the postage. That's right - bulk mail. So if something looks like it's junk mail it is more likely to get tossed. The closer you can make your envelope look like personal correspondence the better since people sort their mail over the trash can. That means you only have only a split-second for them to decide if they'll open your letter or not.