Here's a concept I have seen discussed only briefly, although I know it is in wide use amongst the Elect. Using out-of-game knowledge that the players possess as a feature of the game. Now, I'm not talking here about taking some of the traps of Tomb of Horrors and tweaking them so that a player who's read the module will walk into certain doom if he tries to cheat. That's an old tried-and-true method that any DM worth his salt will embrace and that no player true to his conscience can object to.
But I'm talking about the DM actively using out-of-game knowledge or experience to influence the course of the game. It can be a subtle thing, or it can be overt (to the DM).
Essentially, I am talking about making comments, out of the context of the game, to subtly influence the attitudes, actions, and ultimately in-game decisions of the players.
For example, if the players come up against a particularly nasty monster that has a key role in the plot the characters are following, I might roll some dice immediately before the encounter, make a great show of consulting the random monster tables in the DMG, and then play out the encounter. Even though I knew it was coming all along, and had it planned out in advance, just to throw the players' off the trail. I've had players say "It can't have anything to do with the quest; it was just a wandering monster." Yup, you just keep thinking that.
Or, more subtly, I can present what seems to be the climax to a months-long quest as a relative pushover. The Big Bad turns out to go down in just a couple of rounds, and manages to inflict just minor damage on the PCs. I put a look of consternation on my face as I mutter about saving throws and missed hits, when all the time that isn't the Big Bad at all, but rather a trick to throw the PCs off the scent. If the characters are supposed to believe it, I need to make the players believe it, too.
I've also tried to jack up the nervousness of the players in some situations by rolling dice for no reason, asking "what's your saving throw vs. magic again?" and so forth. Remember that the characters are only as nervous as the players are. If the players are supposed to be nervous (or misdirected, or whatever), playing a little with the players' expectations and emotions is all part of the game.
Songs with Twist Endings - It's always cool to listen to the lyrics of a song and discover that it's not at all what it seemed to be. For most of the song, you are led to believe t...