If you want to cut down substantially on the amount of third-class mail you receive, the best place to start is to write:
Mail Preference Service
Direct Marketing Association
P.O. Box 9008
Farmingdale, NY 11735-9008.
This association will put you on the "delete" list they distribute to member organizations. (Caution: You may stop receiving some of the mail you want.)
Next, call these other large direct-mail generators, take down the address for your region, and write to them: ADVO, Inc. (203) 520-3358; Harte-Hanks (PennySaver or Potpourri) (201) 342-6400; Val-Pak Coupons 800 237-2871; Donnelley/Carol Wright (714) 538-1122.
For charitable organizations that send you unwanted mail, call them up.
Call the customer-service department of your credit card company, and tell them not to share your address.
For unwanted first-class letters, write "refused" on them and send them back.
And unless you want to find yourself on more direct-mail marketing lists, don't bother to answer the questions on warranty or product-registration cards that ask about your hobbies, household income, etc.