Psychological tricks in chess? Are there any psychological tricks that can be used in chess to throw off an opponent regardless of rating?  It also does not matter if it is the opening, middle, or end game.

By psychological, I really don't mean rude/annoying gestures to upset the opponent, but more tricks or tactics that have more to do with making psychological moves to throw an opponent off and they can be specific instances, for example, a specific move for a specific line that might make the opponent wonder "what is going on here...".
Sometimes when your opponent is in zeitnot (English: "time trouble"), it can make him nervous if you repeat moves a couple of times. 

Also when your opponent is in zeitnot, it is a good idea to think on your moves thoroughly and not to try to play as fast as your opponent does. This also applies to when you're in zeitnot; you should at least try to make your opponent play as fast as you do.

I often try to learn rarely used variations to disturb my opponent and make him think with his own brain instead of mindlessly replaying moves from theory. 

If I know that my opponent is rather a strategic player, I will try to play very tactical positions against him and to out-calculate him. When my opponent is better at tactics then I'll try to play more closed, maneuvering positions. In other words, when I know my opponent well, I'll try to play positions that he feels least comfortable in.

I've heard of other tricks of questionable taste, and I personally don't think they fall under "fair play" in my understanding of it. 

Another thing I always do (though it is hardly psychological), is that I never inform my opponent if he forgot to press his clock. I use his time for analyzing the position, and make my move only after the opponent pressed the clock.