This are some of the clues the executive producers of the show (Heller, Gable, Szentgyorgyi, Gable, etc.) have told us about Red John.

- It is very possible that Red John is more than one person. (https://twitter.com/mentalistwriter/status/114527763079245824)

- It would certainly explain a lot if Red John is or was a Mentalist. (http://twitter.com/#!/mentalistwriter/status/114528098116042752)

- Red John has been visible in prior episodes. You’ve got to look for him like Waldo. (http://twitter.com/#!/mentalistwriter/status/114530384280158208) [This comment was made before season 4 premiered).

- Jane is not Red John. (Bruno Heller in an interview)

- "Season 6 apparently they’re going to catch Red John. Seriously, that’s what I heard. I don’t know if I’m allowed to tell you. I don’t know who he is. But apparently we catch him!" (Amanda Righetti in an interview.)

- [Question: Will you cast Red John next season? (this was published before season 5 premiered)] Will I cast him? He might already have been cast. You might already have seen him. (Bruno Heller in an interview.)

- [About the list of 7 suspects] Yes, we can trust that list. As much as much as Red John has been one step ahead, Lorelei Martins’ intervention changed the game. So as much as Red John is trying to play from a position of strength in reality Jane has gotten a lot closer. This last episode of the season really sets us up for next season: The hunt is on, no messing about.

- [About the season 5 finale] This season has been about Jane digging into the notion that it’s not that Red John could be anyone; it’s that Red John is one of a very specific set of people that Jane is uniquely qualified to investigate. The finale is going to pay that off in a way that will completely change the hunt for Red John. We are going to know some very specific things about who he might be. (Szentgyorgyi in an interview.)

- [Question: The arrest or the death of Red John will determine the end of the series?] For me, yes. There will be no follow-up or spin-off. (Bruno Heller in an interview.)

- [Question: Are you spreading clues about Red John's indentity?] Yes. But we're also sneaking in some false clues. Fans will have enough elements to find the solution before the end of the last episode. (Bruno Heller in an interview.)

- [About revealing RJ's identity, before season 5 premiered] Sure, if season 5 we just opened a door and said “tah-dah!” and it was some mid-range actor, that would be disappointing. The trick is going to be — and this is coming — bringing the audience along and making them second guess themselves and ask, “Is that him? Is that him?” Red John ultimately is just a man — whenever you see the great criminals reduced to the flesh it’s sort of disappointing. I have two seasons or so to make it come true. [Joking dryly] I can guarantee that people will be disappointed. (Bruno Heller in an interview.)

That's all I could find so far. If you have any more clues from any of the show producers, please post it in the comments.