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.

Why Patrick Jane isn't Red John? Because the producer didn't want so... On his last interview on Inside tv,  Heller answer that question very cool:
Once again, you’re making moves that make me wonder if Patrick
Jane is Red John — after all, how else could he know that memory from
inside his head? But by saying Red John is definitely on the list, can we rule out the theory held by some fans that Jane is actually Red John?

Heller: I would say that definitively: Jane is not Red John.

Ok now that Bruno Heller has said that Jane definitely is not RJ, and he is definitely one of the seven suspects, it has become a bit easier. http://insidetv.ew.com/2013/05/06/mentalist-finale-season-6-red-john-list/

Gale Bertram is eliminated immediately if Rosalind Harker's words "He is just under 6 ft tall and has short straight hair" are to be believed. I feel RJ will be a character who has appeared in more than 1 or 2 episodes, it makes more sense to allow the viewer to create his/her own perception about a recurring character, and then smash those perceptions. So that eliminates the Sheriff and Reede. 

Could it be Brett Stiles? Maybe. The murder on one of Visualise's farms is an important clue regarding the age of Red John, and it is said the smiley was painted by a kid, barely out of his teens - so RJ would be under 50 years, which rules out Brett. 

Ray Haffner was thrust into our suspicion-list in "The Red Barn" (S05E13) when he revealed he was a Visualise member, he is the right age, and he avoided Lisbon's question about being on the farm at that time. But his voice is very different from that of RJ, and one of the posts here pointed out he wasn't the right height as well. Nor does he have short straight hair. He's out too. 

That leaves us with Bob Kirkland and Brett Patridge. I have a feeling Bob is not Red John, but is somehow a crucial character to the plot. There are several hints that suggest he could be the man, such as his asking to be kept updated on the CBI's investigation in "Red Dawn", his presence when Lorelei's body is discovered, his murdering of Jason Lennon and his subsequent stealth mission to get all of Jane's research. But that video of Lorelei in the season finale last week changes the game completely - it means RJ knew about Jane's suspects BEFORE Kirkland raided his room, which suggests Kirkland is not RJ, but is obsessed with him for some other reason. 

Therefore by process of elimination, Brett Partridge seems to be our man - but I have to add, it's a disappointment. While he seems to love the atmosphere of crime scenes, he also has the right voice, he is the right age, he has access to the CBI because he works there, but as a character, he seems too small and insignificant. Red John knows a happy memory of Jane's, and also knows his seven suspects, and that points to a seriously resourceful and brilliant villain. Jane treats Partridge with disdain all the time, and he has had very little screen time too. But he IS a recurring character, and maybe the clue for us in the sixth season will be to see which of these seven characters are subtly being focused on. 

There is however one possibility that could bring Kirkland back on top of the suspect list - what if Lorelei's murder was actually staged? Jane doesn't really examine the body, he just walks away after a few seconds. What if the video with Lorelei was shot AFTER Kirkland raided Jane's room and assembled his own copy of Jane's board? That and the fact Jason Lennon was murdered by Kirkland would make it pretty damn conclusive. 

But if the writers have faked one clue, it casts a doubt on other clues, such as RJ's height and hair, which would bring Bertram back into the game. Also, regarding the age of RJ - could he have merely adopted the smiley face which was drawn by some random kid on the barn? Even if he had murdered the two guys, he could've looked at the smiley face and got the idea of marking his killings that way. By this reckoning Brett Stiles comes into play as well. 

I would love to see that happen, so that we could have a rethink on who Red John is, and clues from the sixth season will prove to be decisive. But for the moment, I'm convinced that it will be Partridge, and that for me, is a disappointment. 

"Happy memory" thing is easy to explain in mentalism way. (Almost) everyone has that friend from the past which reminds of some "good old days". In this case it's childhood. Red John has Patrick very well researched, so finding such person wasn't a problem. It's plausible (like everything in the show) and you don't need any mind reading skills to do that.

My theory (or should I say hopes) about Red John is that he's Brett.  Everything else would be disappointing, because I don't think that Heller and producers can come with something that will suprising and... good. I hate that Red John evolved from normal serial killer to some Moriarty style villain with hundreds of accomplices who are ready to die for him without any hesitation, and I'm afraid that ending could be so twisted that it will be just overdone (i.e. "Lost", clearly writers just didn't know how to end the show and resolve that tons of mysteries!).

And one funny thing about season finale - why the heck they transport such a valuable convict like that woman from social services who was RJ puppet in such ordinary way? If they knew how important this person is and about suicidal tendencies of RJ friends (and they clearly did know!), they would never let her be transported only with one cop and without any protection from commiting suicide. "Mentalist" has often problems depicting CBI and police work...in real life there's no way that they would let so many witnesses so easily die. There's offcourse more lapses such as how it's possible they didn't pursue the plane from "Red John's Footsteps" (I think they could easily get to this plane and the pilot) or didn't have RJ on tape when he come into CBI offices or around the building (in one episode there was a footage he did from his car - at least they should try to find that car)? There are no cameras around (he was also in a shop stalking Darcy)?