Wanna get your sales letter thrown in the trash? Make it look like junk mail. A plain white envelope with a metered stamp and will do it every time.
But if you want people to actually open and your mailer, be creative. Use a first class stamp. Put the stamp upside down. Print the address and return address in different color ink by hand. Write "Important Information" or "Time Sensitive Material" on the front. Your prospects will open your mailer every time.