Buy Dark World Stuff :
La Pucelle: Tactics
Go to the Item Shop and buy every item available. The Shop will restock with Dark World Items.
Triple XP, Money, or an Effect :
Visit the Rosequeen Shop in Pot au Feu City and purchase some items (preferably cheap ones as this process tends to be lengthy). Fill out the survey (in any fashion) and then sell the items you just bought back. Repeat until the shopkeeper asks "If you had one wish, what would it be?" Now you can get Triple Effects. Note: Triple XP applies to the next battle you engage in or the next Dark Portal you purify.
Thanks to Revolution readers jdogg, c00ki3m0nst3r, and Raul.Stevens!