Easy Money
After giving O'aka 100,000 Gil to pay off his debt, he will sell you items for extremely cheap prices later. He sells Hi-Potions for 50 Gil, but you can still sell them for normal price.

If you buy 99 Hi-Potions at the 50 Gil price, and then sell them all, you can make over 3 times the money back. You can do this process over and over until you have the desired amount needed.

Easy Points
As long as the Gunner Gaunlet's Minigame is available, you can do this trick anytime. For an easy point in this minigame, go below the last waterfall to avoid conflict with the approaching enemies. Kill them and score points as you desire.

Inflicting massive damage (US/UK Version only)
When your in the Via Infinito, keep going through it until you fight the Black Elemental. When defeated, you will get an accessory called Cat Nip, which has the Auto-Ability SOS-????. Heres the good part, if a character with this equipped is reduced to a critical state, then all of her attacks will inflict 9999 damage, combine this with Trigger Happy and you can deal MASSIVE damage to the enemies. This works really well on several bosses.

Longer trigger happy
Have someone use haste on the person with the gunner dress sphere. Then trigger happy will last longer.