Set the system date to your birthday and make sure the day and month match the date that you set as your birthday in the game's profile. Load the game and do any two jobs. Loading a job then exiting it does not count; you must play it until getting a game over. Go back to the main menu and check your e-mail(s). One of the subjects will be "Happy Birthday". Read that e-mail to get $50. Exit the e-mail and the $50 will be added to your total money. Delete the e-mail, quit the game, then change the system date's year to something else and start the game again. Repeat to get another $50. Do not forget to delete the e-mail each time and change the year in the system settings. You can get lots of money this way very fast.
-Set the system date to December 25th to get $20 in an email. Set the system date to January 1st to get $30.