There is something called the "one at a time" rule. From Robert Hickey, deputy director of The Protocol School of Washington® (PSOW), has been conducting protocol trainings and workshops since 1988. , who are experts at this type of etiquette: http://www.formsofaddress.info/former.html

This is the traditional approach for any office of which there is only one office-holder at a time. So, with officials such as mayors, governors or presidents ... only the current office holder is addressed as Mr. Mayor, Governor, or Mr. President ... formers are not officially addressed that way. That's not to say some reporter might not call a former mayor Mayor Smith or a former president President (Surname) ... but formally and officially these are incorrect. With offices of which are many office-holders at a time ... senators, admirals, judges, etc. ... former office holders DO continue to be formally address in the former style, since it's not disrespectful to a singular, current office holder.
To explain the correct form I would say "using the title of a former position is flattering to the former official and he or she may not correct you, but is not respectful to the current office holder. There's only one "(name of the office)" at a time." -- Robert Hickey

and

Our former Presidents of the United States are most formally directly addressed as Mr. (Name) ... not as President (name) ... and they are identified as "the former President of the United States". You will hear the media say President Clinton in a news story to be clear who is being discussed, and because the media does it, that form is widely thought by many to be the correct form of address. But it's not the traditional form. The correct form for formal introduction -- e.g. from a podium before his speech to the audience would be something like ... It is my pleasure to introduce The Honorable William Jefferson Clinton -- but you wouldn't address him that way in conversation. In conversation address him as Mr. Clinton. In an introduction to just one person you can say Mr. Clinton may I present... -- Robert Hickey

Also, from an Answers.USA.gov site here

If you wish to contact former President Jimmy Carter, you may do so at:
Office of Jimmy Carter
The Carter Center
453 Freedom Parkway NE
Atlanta GA 30307
When sending letters to former Presidents, the proper form for addressing the envelope is: The Honorable (President's name) The proper form for the salutation in the letter is:
Dear Mr. (President's last name)

EDIT: I suck at formatting, sorry