One of the best ways to mentally prepare is to use mental visualisation. Find somewhere quiet, without distracts, where you can lay down but won't fall asleep. Now close your eyes and picture yourself in your mind, strong, fast, agile, ready to fight. See yourself in your mind's eye. Breath slowly and calmly through your mouth, to attain a deep relaxed breath throughout your diaphragm.
Next imagine that you are at the karate competition and you are facing your first opponent, bow to the referee in your mind, then to your opponent and finally begin, see yourself throwing combinations of techniques, each landing on your opponent, imagine doing this when he is throwing combinations of hand strikes and kicks, see yourself blocking, evading, avoiding and counter-striking. Do this for however long you feel is necessary. Then imagine the referee taking your arm and announcing you as winner.
Do this every day leading up to the tournament. You are a winner. What do winners do? They win.
Next imagine that you are at the karate competition and you are facing your first opponent, bow to the referee in your mind, then to your opponent and finally begin, see yourself throwing combinations of techniques, each landing on your opponent, imagine doing this when he is throwing combinations of hand strikes and kicks, see yourself blocking, evading, avoiding and counter-striking. Do this for however long you feel is necessary. Then imagine the referee taking your arm and announcing you as winner.
Do this every day leading up to the tournament. You are a winner. What do winners do? They win.