White Turnip Profits
Every week on Sunday morning between 6 a.m. and noon, real time, Joan the boar will visit your village and sell wares. She sells white turnips for a price of around 100 bells. If she's selling turnips for anything under 130 bells, buy as many as you can. Deplete most of your bells to buy them. Empty your inventory and place the extra turnips in your home if need be. Over the next week, visit Tom Nook the tanuki in the town shop. See what price he's willing to give you for turnips. He offers two prices per day, one before noon and one after noon. Sell all of your turnips whenever he offers more than you paid for them. Do not keep the turnips beyond Saturday, as they will rot.
Calendar Cheat
Holiday items tend to be worth more than the usual "Animal Crossing" items because they are available only once a year, real time, when it's a holiday. Halloween and Christmas furniture especially sell for a lot of bells if you sell them at Tom Nook's town shop. To quickly grab these items, log onto your Wii's System Settings menu before you start the game. Input the date of the holiday in the past year and log back into the game. The game will act as if it is the holiday and you will have access to rare holiday items to sell. Set the date back to present day in order to resume normal game play.
Sell to Friends
Friends who play "Animal Crossing: City Folk" will be able to access your town via a Wi-Fi connection. Offer to sell rare items to your friends, especially friends with a lot of bells to spare. You can use a calendar cheat in order to procure rare holiday items that your non-cheating friends can only find once a year.
Time Travel
Another useful calendar cheat is to take advantage of interest on your bank account. Put at least 50,000 bells into the bank and then change the Wii System Settings calendar to any date in the year 2000, the earliest year it will go. Start up the game and immediately log out of the game. Change the Wii Calendar to any date in the year 2035, the latest year it will go. When you start up the game, you will notice that your interest has earned you 100,000 additional bells.