Any letter is better received when it is addressed properly. Letters to former presidents are no exception. Yours can hit the target when properly addressed.
Determine which former president you wish to contact.
Find the appropriate address. Generally, these are available through each former president's website or the site of their presidential library.
Address the envelope using standard conventions. “Honorable” is a title often used for retired high-ranking officials, so the envelope could be addressed to "The Honorable" and his first and last names. For example, "The Honorable Jimmy Carter" or "The Honorable George H. W. Bush."
Begin the letter. Although style guides vary, when addressing letters to former presidents, it is appropriate to use “Mr.” and his last name. Therefore, the greeting to the letter would be “Dear Mr.” and his last name. For example, "Dear Mr. Clinton" or "Dear Mr. Bush."
Continue writing the letter in a respectful and succinct manner. When you are done, sign the letter, seal it in the envelope and mail it.
Items you will need
If one were introducing a former president, it would be appropriate to introduce him using his title as “former President” and his last name, but this is generally not done when addressing the man, directly. Note that “former President” would be preferable to “ex-President“ in this circumstance.