“Lie until even you believe it - that's the real secret of lying”

Source: The Last Song

Observation made privately, quoted by Time journalist Michael Kramer, The Case for Skepticism http://www.time.com/time/magazine/article/0,9171,956604,00.html Time, (26 December 1988), in the context of doubts about PLO sincerity in hinting about recognition of Israel.
1980s

Compare sourced quote set forth above: "The English follow the principle that when one lies, it should be a big lie, and one should stick to it."
Attributed to Goebbels in Publications Relating to Various Aspects of Communism http://books.google.com/books?id=iLAnAQAAMAAJ&q=%22If+you+repeat+a+lie+often+enough,+people+will+believe+it.%22&dq=%22If+you+repeat+a+lie+often+enough,+people+will+believe+it.%22&hl=en&sa=X&ei=U4gPUvObG4qMyQHlhYAw&ved=0CGQQ6AEwCQ (1946), by United States Congress, House Committee on Un-American Activities. No reliable source has been located, and this is probably simply a further variation of the Big Lie idea.
Variants:
If you repeat a lie often enough, people will believe it.
If you repeat a lie often enough, it becomes the truth.
If you tell a lie big enough and keep repeating it, people will eventually come to believe it.
If you repeat a lie long enough, it becomes truth.
If you repeat a lie many times, people are bound to start believing it.
Attributed in The Sack of Rome (2006) by Alexander Stille, p. 14, and also attributed in A World Without Walls: Freedom, Development, Free Trade and Global Governance (2003) by Mike Moore, p. 63.
Misattributed

TEDx talk at Oberoi International School, Mumbai https://www.youtube.com/watch?v=ZUisKJ7hnwA

Harry Potter in Ch. 65 http://www.fanfiction.net/s/5782108/65/Harry_Potter_and_the_Methods_of_Rationality
Harry Potter and the Methods of Rationality (2010 - 2015)
Context: Lies propagate, that’s what I’m saying. You’ve got to tell more lies to cover them up, lie about every fact that’s connected to the first lie. And if you kept on lying, and you kept on trying to cover it up, sooner or later you’d even have to start lying about the general laws of thought. Like, someone is selling you some kind of alternative medicine that doesn’t work, and any double-blind experimental study will confirm that it doesn’t work. So if someone wants to go on defending the lie, they’ve got to get you to disbelieve in the experimental method. Like, the experimental method is just for merely scientific kinds of medicine, not amazing alternative medicine like theirs. Or a good and virtuous person should believe as strongly as they can, no matter what the evidence says. Or truth doesn’t exist and there’s no such thing as objective reality. A lot of common wisdom like that isn’t just mistaken, it’s anti-epistemology, it’s systematically wrong. Every rule of rationality that tells you how to find the truth, there’s someone out there who needs you to believe the opposite. If you once tell a lie, the truth is ever after your enemy; and there’s a lot of people out there telling lies.

Source: To All the Boys I've Loved Before

Examples of self-translation (c. 2004), Quotes - Zitate - Citations - Citazioni