Tips&Tricks
It is not sure that everything on this page works. It may be Spoilers. If I know that the trick does work, a * will stand at it.
 
 

W-Item Trick*
Equip W-Item (Found in the sewers after paracuting into Midgar) and then choose one item. Then choose the item you want to get more of(Cirkel), and then press X. Press Cirkel and then press X again and again and again.....

W-Summon Trick*
Equip W-Summon (Found at Battle Square)
You can now use two summon magics at the same time.

Wood Chocobo
Go to fight Lord Godo with Yuffie. Steal his underwears.
Go to a forest with them, and a Wood Chocobo will attack you.

Soldier Armor
Get 13 1/35 Soldiers, and then go to Mr.Hangman in Gost Shop. Sell them to him. He will ask if you want the hat or the armor. Choose the armor, and equip it. In batle, the one with the armor on will look like a Soldier.