Coin flips are not random and happen in a pre-determined order. To exploit this, use an attack, trainer card, etc. that involves a coin flip. If the flip doesn't come up what you wanted, immediately turn off the game without leaving the screen that shows the coin. Turn the game back on and select Continue Duel. The duel will be where it was immediately before you had flipped the coin, but the outcome of the very next coin flip will always be the same as the one you just saw. This trick is extremely useful because you can keep from wasting trainer cards or attacts if you flip tails, and by skipping your turn you can even guarantee that your opponent's next flip will be tails.

Note: Do NOT try this trick on the coin flip that determines who's turn is first, as your game will NOT save.