Moshi Monsters' economy and mini-games have changed over its lifespan (and the original game is now offline). Because I don't have real-time access to private servers or fan revivals, I can't give a current "best" method. However, these principles applied to the live game and typically hold for similar virtual-world titles:
Warning: Avoid "Rox generators" or account-sharing schemes — they're scams that steal credentials. Play fair; the grind is the game.