Side girls can be arranged by geography. As stated previously, in less polished vernacular, "having hoes in different area codes."
Other playboys may choose to keep his girls on the side by specific traits or functions they may perform.
Sometimes, although rarely, side girls can be promoted to main girl status. However, this is typically frowned upon. It's important that side girls know their place in the sociological realm of their male counterpart, or else they can become apparent to the male's main girl, a situation which could make a side girl a main girl, in a pinch.
Jimmy: The blonde with the big tits?
Jimmy: Ohh, that's Amy, she's my side girl.