I usually compare a duel with a game of chess, lose your focus, lose the match. Never let your opponent see what you will do, or what cards you have to use. Even if your hand is a mess, there is always a chance to turn the tables.
A letter, a mistake, a wrong activation. All this and more are some causes for a victory or a defeat. Never give up on a game by more than his defeat is certain. Remember that it will only be right if your life points go to zero. Otherwise, there is always hope.
And when you're in command of a duel, never let your opponent have a chance. If you have a chance to detonate the points of his life, do it, because just as there is a chance of facing you there for him.
I've seen good gains duels duelists lose by leaving the opponent with 500, 250 and even 50 hit points.
When mounting their decks keep in mind two things: never leave him dependent on a single letter or chance. For thus you will see the defeat more often than victory.
The other thing to bear in mind the balance of it: 20 monsters and 20 magic or trap cards are more than enough. The other possible choices should be on your side deck.
For now, that's all. Any doubt in my Office to post or send pm here or DN.