Just take fine aim at a citizen for a few seconds; eventually they will drop their money and run away. Rinse and repeat 50 times. (Don't try mugging cops, rival gang members, or your own gang members; it won't work. Also note that if a cop is close by when you try to mug someone it will increase police notoriety and the cop will attack you.)
This will also unlock Jane Valderama as a homie. Her sayings when she's with you are hilarious.