Look through your deck without it reshuffling
To do this, you must have a Trainer card in your hand that enables you to look through your deck, to pick a certain kind of card, and then reshuffle the deck. Use this card, and then, instead of selecting a card, press A+B+SEL+START all at the same time, once you are done looking at the deck. This will Soft Reset the system. Choose Continue Duel from the menu, and you will get back into the duel with the same cards you had before you used this code. If you used Computer Search or another card that takes cards from your hand to look through the deck, those cards will be back in your hand.

Get A Promo Lv.9 Slowpoke
Take 59 of your ENERGY cards and 1 basic pokemon and make a deck with them. Select the deck as your dueling deck. Then go talk to the boy in the Fire Club. He will ask you to give him all your unused ENERGY cards. Tell him Yes. If you tell him No, he will go away and not come back. Then you can't get the Slowpoke.

Infinite Energy Cards
If you are low on energy cards and need more, go back to Dr. Ooyama's lab. There fight Mikasa (The man you fought in the beginning)and fight him again, again,etc. getting 2 boosters full of energy cards each time! Now you don't have to worry about if you don't have enough energy or not!

Free Booster Packs
Go to a computer in one of the gyms. Find the menu that has japanese writing and then numbers next to each one. If you click on any of those items. You get a booster pack every time!

Get Charizard
After you start a new game and beat your instructor, talk to Dr. Mason and ask him to build you a Charmander and Friends deck. He'll put a Charizard into it