You buy territory.
The more territory you have, the more income.
I'm currently a level 63, with 462 mobsters
with $47,772,000 hourly income
$12,527,500 upkeep leaving me with
$35,244,500 an hour.
Mike, if you wanna get more mobsters(which are very important the higher level you are) check out sumob.com. You can add a s*** load of strangers, add them to mobsters then delete them off your myspace.