This trick requires the Fairy Bank and the Foreign money policy of Mils. Buy Mils when they are very low (for example, 85-110Z). Then, fight a few battles and return. If the Mils are not worth over 180Z, fight more battles until they are. You can start off with 30,000Z and end up with 110,000Z.
Helping the shopkeeper
During the game you will meet a man who wants to set up his own shop. Depending on how much you give him, the shop will have different wares:
1-100: Basic weapons.
101-500: Basic skills
501-1000: Better weapons and some skills.
1001-2000: Better skills, but more items then skills.
2001-2500: Mega heal kits and such at low prices.
2501-3000: Your choice of good weapons, items, skills, armor and a mini-game.
3001-5000: Great weapons for Lin, poor weapons for Ryu and Nina, and good spells for Nina.
5001-10000: Your choice of great weapons, skills, spells, stat increasers, armor, items, but no mini-game.