staying "on the grind" means to work hard, always be hustling, or otherwise engaged in money-making or woman-procuring activities.
people who were once poor and become wealthy might then become lazy and indolent and spend all day in champagne-filled jacuzzis whereas other newly-successful people might continue to work hard, thus "staying on the grind"
there are many examples from 50 Cent's latest release, "the Massacre", but i'll offer up two:
excerpt from "God gave me style":
A O.G. told me God's favorites have a hard time
You out the hood, that's good now stay on the grind
Chorus from "Bitch get in my car":
I got no pickup lines
I stay on the grind
I tell the hoes all the time
Bitch get in my car (Bitch get in)
I got my 64, ridin' on Dayton spokes
And when I open that do'
Bitch get in my car