Go to the desert outside of the golden saucer and walk around for a while. Sooner or later a little cactus man will appear. You can't beat him with magic or weapons, but he can be defeated by summoning Chocobos or Mogs. If you manage to kill him he's worth 10,000 gil!

For better results, head for the Mideel Area (an island in the south) and look for Sea Worms. Each one you defeat is worth 1300 EXP, 200 AP and 5000 gil!

To do this trick, you need a joystick controller with rapid-fire settings and access to the Gold Saucer.

Go to Wonder Square, and up to the arm-wrestling game. Wrap a rubberband around your joystick so that it's pulled to the upper left diagonal, so that Cloud continuously walks into the arm-wrestling machine but not into the GP-trading girl. Now, turn on the rapid-fire for the 'O' button. Cloud will now repeatedly challenge and beat the wrestling game. Slowly but surely, your GP will increase. It takes a while, so it's probably best to set this up just before you go to bed at night, and leave it on while you sleep. By morning, you should have plenty of GP to buy your Gold Pass and the two secret Materia: XP-Plus and Gil-Plus. Cheater!