"The greatest fear of mankind is fear of the unknown"
HP Lovecraft (albeit paraphrased I think)
I guess this is kind of echoing what others have said already, but what the hey.
I second the notion of denying the players the chance to look things up. To this I add:
1. Keep track of the damage that their PCs suffer. I picked that tactic up in some old Ravenloft material. It's one thing for players to say, "Oh, my fighter's down to 2 hp...I'm retreating." It's another to have the DM say "Your fighter feels that he can still stay in battle, but those wounds are looking really nasty." Uncertainty rocks.
2. Speaking of which, never underestimate the value of the word "appears". As in, the door appears clear of traps, the dragon appears willing to talk, etc.

Runners up, "apparently" and "seem".
3. Be cryptic. In the first season of Twin Peaks, someone said "The owls are not what they seem." Not sure what it meant but it creeped the hell out of me.
Hope these help. As you can see, all of them still trace their roots back to "Leave the PCs in the dark. Take away their flashlight. Enjoy."