Psychology, along with Deck Building and Luck, makes up the trio of what makes a good duelist. I'll post articles relating to the other two later.
DMO has a few places where psychology can be implemented. The first you'll meet is the game before the game; Rock, Paper Scissors. When playing at tourney-level you'll notice many duelists don't pick randomly. Each is useful against a certain kind of opponent. Being able to tell what your opponent will pick allows you to go first (Going first is more-than-often optimal). You'll find:
Newbies and Pros use Rock. Newbies as it is seen as powerful and the first thing that omes into their head, Pros as many intermediate players use scissors.
Newbie-Intermediates use Paper. They're so experienced with using rock and others using rock, they'll adapt at this stage and start to use paper.
Pro-Intermediates use scissors. It's a naturally sneaky move, due to many player one step below them use paper to counter those one step below themselves. Players of this level often like the idea of a double bluff.
Predict your opponent's skill before you duel them, prehaps by looking at their Unlocks/Bonuses or a little conversation.
More than a third of the time, you'll end up in a drawing situation. Again, judge their level of skill. If they're newbie, they'll often go for the same thing, not bothering to change and believing in sheer luck. Newbie-Ints will attempt to counter what you just did, taken from experience of being a newbie. Pro-Ints will occasionally go for the double bluff and pick what you would of won last time. Pros usually go for the double bluff, as the triple bluff is too risky. The third time round, against pros, try the double bluff again. If you go for a third time round, go for a triple bluff.
And before you say this is a pile of crap, ask yourself why I come out top on RPS 80% of the time:P
Psychology in dueling is similar. For example, if you don't want your opponent to attack, you can sway how they feel with words, if they're a newbie. When playing newbies, and you don't want them to attack, try telling them to attack. They'll assume you're bluffing, thinking they're clever, when you're actually double-bluffing.
A form of psychology I've come to hate is the intimidating kind, now. It's completely useless at Pro level and will actually make them more confident. It's makes you look like a jerk, and loses you friends.
And finally, end-game-pyshcology. Whenever you lose, you didn't lose because you were unlucky. You lost because your deck isn't the best deck ever. The best deck ever couldn't possibly lose. That means you cna improve; and denying the fact you lost fairly is denying yourself the opportunity to improve and learn from your mistakes!