I play it with a poker face. A ploy I might use over the board if I know the opponent is looking for any hint of what I'm thinking is to concentrate my gaze somewhere particular on the board, I already know what I'm going to do and it has nothing to do with where I'm looking. It's misdirection.
Give the opponent some time to fret about it and maybe he misses the obvious by looking for a weakness in his position that isn't there. It's hit and miss.
The most effective of all psychological ploys are the ones that are below the level of consciousness, aka hypnosis. Josh Waitzkin talks about it ( I believe in his book "The Art of Learning" if my memory serves me right) He had multiple encounters with a dirty opponent that liked to tap one of the captured pieces against the table when Josh was in a critical position and thinking of his next move. This caused Josh to make a hasty move without realizing what was happening. He only learned about this after somebody informed his father about it. I notice a similar effect when I shift in my seat or change arm positions. All of the sudden my opponent moves and I don't think it's a coincidence. I don't do it on purpose but I do notice it. I do however use this knowledge when my opponent is apparently oblivious about his time trouble and so I make sure to not move a muscle.
My personal decision on when to make a move during a tournament I do think is substantially subjective (maybe I should work on that) but because of this I believe their moving, sitting, standing, leaving, etc has no effect on when I move. When I finally make a move, I have to feel psychologically good about it which comes only after checking many possible responses, and having a relatively positive evaluation of the final position. I have strong negative feelings about making fast or improperly calculated moves at a tournament because in the past I lost games after (it seems to me anyway) 100% of my quick moves turned out to be blunders.
Now when I blunder, I'm determined to make it a real lesson with no excuses, never something I can write off as "well I wasn't paying attention that moment/game/day/etc" and actually blunders that have no excuse make my games more precious to me (not just as lessons)... Anyway I'm getting off topic now...