Instructions
Deposit all your bells into your bank account by speaking to Pelly or Phyllis at the Town Hall, selecting "Access Account" from the menu, selecting "Save Bells" and entering the amount you want to deposit. For the best results, aim for a balance of about 50,000 bells. To earn bells quickly, gather fruit, catch rare fish and insects and sell unused items to Tom Nook at his store.
Remove any items from your mailbox next to your house, and save your game. Reboot your DS and select the "Settings" panel from the main menu with your stylus. Select the "Clock" option and adjust the year to 2099 using the "Up" and "Down" arrows. Select "Confirm" to save the new setting.
Launch "Animal Crossing: Wild World." When you open your mailbox, you will receive the interest accrued on the balance in your bank account. Depending upon the amount originally deposited, the interest can range from several hundred thousand to over a million bells.
Save your game and reboot your DS. In the "Settings" menu, select "Clock" and readjust the system time to the correct current year. Select "Confirm" and return to your "Animal Crossing: Wild World" game. Repeat this process until you have acquired the desired quantity of bells.