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 A Promo Level 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!
Easy Booster Packs
Just after you pick your first deck of cards, go to your Weakness gym and fight the losers. Since they are easy you should beat them, and they give you two booster packs just as everyone else does. You can get awesome cards just after about 2 duels.