Emily Post says:

When addressing a former President of the United States in a formal setting, the correct form is “Mr. LastName.” (“President LastName” or “Mr. President” are terms reserved for the current head of state.) This is true for other ex-officials, as well. When talking about the person to a third party, on the other hand, it’s appropriate to say, “former President LastName.” This holds for introductions, as well: A current state governor is introduced as “Governor Tom Smith,” while you’d introduce an ex-governor as “former Governor Jim Bell.”