Subtlety. Make it look like news articles if you post it in newspapers. Make people blog/vlog about it (and depending on what kind of a movie it is, have them disappear/change/whatever). Something like what they did on The Truth About Marika (http://en.wikipedia.org/wiki/The_Truth_About_Marika) would be awesome as well. In short the plot was divided into two parts, the one that aired on tv, and the "real" main one, that played out online, and in the streets of Sweden, in every town. It started out as a simple forum, and a blog. A girl wrote about her friend, that got kidnapped, she asked for help finding her. They found more and more information about it, and it turned out to be a big conspiracy, people were divided into cells, given tasks to accomplish to find out more about this company, and this security firm. Up until this point no information was given to indicate it was fictional on the official site. After a month or so the tv-series was announced as well, and ran parallel with the plot of the game. I had a friend who got really into that thing. Like, really into it. With one day left she was talking about either quitting or comitting herself to a mental institution, she was in a complete disarray, she couldn't separate reality from the game, at all. She didn't trust her friends, she trusted the people from the program more, one of them, (hired actress) escaped from the evil company and spent the night on her couch, she freaked every time she saw the security company's cars. It was very intense, and very real, and very subtle, and that's why it worked.
Sure, it sounds bad, but it was really cool, even she admitted that, when she got home, and they told her very firmly at the finale what was real and what wasn't.
I also saw a very neat (but unfortunately, a bit broken) website that was supposed to advertise some book, or movie or something about a scary hospital with connections to hell. It started out completely normal, and sometimes there were some small things that were just wrong. Some names, something moving in the background, some subtle sound effects (real sound over, creepyness almost inaudible below). But unfortunately some things were just way over the top with a page that was on fire and saying stuff like "Hell is here" with lots of blood, and then the illusion just broke.
As with the movie, you need to work with suspense, and build the atmosphere, you need to make it real, subtle, and believable. Decieve people, trick them, don't tell them anything directly. Make them find your movie, instead of shoving it down their throats with Americanesque (explosions, dramatic voiceovers, lots of flashing text, lots of very overly dramatic and poorly written oneliners) trailers.