Put any amount of money into your savings account and once there are about 50,000 bells in your account, save the game and turn off the Nintendo DS. Go to the DS’ system settings and change the date to the year 2099. When you resume the game as the same character, you’ll receive a letter from the Post Office stating that you have around 99,999 bells. This money is already in your savings account. Note: You will get many weeds from doing this.
If you have more than 700,000 bells, you can probably get 99,999 bells in just about a year by changing the Nintendo DS system date to the year following. You will eventually receive a letter in the mail saying that you have a certain number of bells. If you are rich then this will usually yield 99,999 bells.
While your player is sleeping, select the phone and choose "Set Date And Time" and change the date to January 1st. You can repeat this any number of times by setting the date to a year later, though you will have a lot of weeds.