Leaks and from the press and IMdB.
To be perfectly honest, I would have enjoyed this show a hell of a lot more if I had discovered it after it was finished, had heard nothing, and binge watched it with no contact with the outside world until I was done. So many things have been diminished for me because previews and the like have given them away, like the Viper fighting the Mountain. I would have loved to have been clueless, wondering who was going to defend Tyrion, until Oberyn made the epic reveal, but that couple of seconds from the season preview ruined that completely, and the scenes with Jaime and Bronn as well, knowing it wasn't going to be them. I go way out of my way to avoid stuff like that, but someone posted a gif of that fight right in this thread, so I was burned.
I don't understand why there isn't a rule about putting spoiler tags around anything from previews, books, casting or the like. It would be insanely easy to enforce and would fix so many problems. People who want to see it could just click a button, people that are only here to discuss what we've seen or THINK will happen would be safe.
It doesn't affect me so much any more because I just said screw it and buried myself in the lore, but if there's ever another mega show like this again, there is no way I'm going to spend any time talking about it in a Sherdog thread until it is completely over.