Wouldn't it be better to just study the game more rather than worrying about how to psychologically mess with your opponent?
Psyching out my opponents - half way to winning already.

It's better that your chess playing tool-kit is fully and extensively stocked. Of course study is important too but victory is about the accumulation of incremental gains.

Of course it would be unfair and not in the spirit of chess to use these advanced techniques against the juniors.

I find the best tactic I have used in school tournaments is to conveniently find all my previous school trophies in my bag, and place them on the table to admire them during the games.
Even if this doesnt help at all, I enjoy it.

everytime they want to move scream "noooooo bad move" at the top of your voice.
start naming everything they do outloud: breath in, touch pawn, breath out, move hand upwards.
when they dont look switch your king with a queen(guaranteed win)
when they have won, simply turn around the board and keep saying you've won.
thats all i've got for now :P

Personally, I favor wearing a large fake scar prominently. I practice leering as much as I practice playing chess, so I have become proficient at this, and I put it to good use every game. I also fiddle with a knife (the bigger the better) between moves while they're thinking, and occasionally cut the heads off of pieces that have been removed from the board.
Furthermore, I occasionally make a move in seconds in immensely complicated situations after they just spent a good twenty minutes thinking on their previous move. That really psyches them out.
Also, if at a critical moment during the game they make a brilliant move which would put me on the ropes I lean forward and whisper "Are you SURE you want to do that?" and raise one eyebrow in a threatening and meaningful way. Having an accent is a bonus here, so I suggest practicing one if it doesn't come naturally for you. This is also a good time to fiddle with your knife.
Hope that helps!

Don't shower for two days befor the tournament, then kick your shoes off under the table.
I have a similar technique involving my particularly stinky slippers. Its suprisingly effetive during live games...

dc1985> That seems very unsportsman-like
Well, the Beast meant this in jest, but I wouldn't worry about this happening in a real game. Those "!!!s" alone would likely get you the point in a USCF game.

one chess giant proffered the powerful notion of eating a big, spicy, smelly pastrami sandwitch. a powerful idea indeed.

In Wales we are born hard and we play hard.
In the bible God made it rain for forty days and forty nights.
That's the best summer I can remember in Wales - the hose-pipe ban - I thought it'd never end. From this bleak upbringing comes fortitude. I was thirteen before I realised you could take the Kagool off - so don't talk to me about fair.

Personally, I favor wearing a large fake scar prominently. I practice leering as much as I practice playing chess, so I have become proficient at this, and I put it to good use every game. I also fiddle with a knife (the bigger the better) between moves while they're thinking, and occasionally cut the heads off of pieces that have been removed from the board.
Furthermore, I occasionally make a move in seconds in immensely complicated situations after they just spent a good twenty minutes thinking on their previous move. That really psyches them out.
Also, if at a critical moment during the game they make a brilliant move which would put me on the ropes I lean forward and whisper "Are you SURE you want to do that?" and raise one eyebrow in a threatening and meaningful way. Having an accent is a bonus here, so I suggest practicing one if it doesn't come naturally for you. This is also a good time to fiddle with your knife.
Hope that helps!
Lol. I've never had the ignorance to try any of this stuff, but one of these days I might.

I've not tried your particular technques, although I can see the benefits and as a fellow Welshman am tempted by your Haka as a method of preparing myself for battle.
I myself have a particular favourite, upon each move I gring my piece into the board as if boring a hole, I find this unsettles my opponent as he fears for his own safety should the match turn in his favour.
As a counter to your technique of straightening your opponents pieces, I deliberately not only place my pieces on the edges of their squares but quite often face them backwards - this is an extremely effective technique that I would recommend to all enthusiastic chess players.
Finally, and not something I am proud of, I once found myself in a losing position against a Jewish boy and remembered seeing a packet of Frazzles (bacon shaped and flavoured crisps) in a vending machine upon entering the foyer of the competition venue. To my horror I went and bought a packet during the game and offered him some - this did not go down too well, especially with his father who was standing behind him. Althoug I ended up wining the game I am forever carrying the guilt of what I did so shall continue with grinding and reversing my pieces only from now on.

I just tried to psych out an opponent on MSN chess. Unfortunately, I don't see a future in messing with online opponents. I spent so much time typing it was worse for me than him. I lost but maybe I wasn't doing it right. Further experimentation is merited. My method consisted simply of making an insulting comment after each of his moves. My comments included "BAD MOVE, resign now and retain you dignity and yeah, I like that" just to name a few. Are there any suggestions for online techniques?
In Tournaments after I've done the Welsh Haka I like to greet my opponent with a full head twitch, which I can then continue intermittently and with increased vigour and regularity through the duration of the game. It's very off-putting (especially if I hold the table hard as I'm doing it - having to reset all the fallen pieces can be so exasperating after the third time) and gives me the psychological edge from the outset.
The opponent usually feels too awkward to complain.
Another technique that I use alongside this is that after every move they make I touch the piece that they have just moved to ensure that it is exactly central on the square (on both axes) and that it is facing exactly forward as I do this I say "J'adoubeee dooo".
The final part of my genius tri-partate strategem is in the notation. After every single move I make I finish my notation with three exclamations (e4!!!) which I write with a fluorescent marker pen (the squeakier the better). When I write my opponents moves I always put three questions marks after it (...e5???) again with a bright orange marker pen.
I have yet to lose a tournament game where I have employed this triumverate of killer techniques proving that chess is indeed a very psychological game.
Does anyone else have any interesting pysch techniques that they could share?