You can join up to three jobs at a time by default (there are extra job tokens out there, but that's not for this guide!) and as each job improves in level, you will earn more money for those jobs. But be warned! If you swap jobs back and forth (leave one job to join another) your experience in the job you're leaving will be lost! This means that if you rejoin that job later, you've gotta start the grind all over again!