Never been late and have $350K line.
Just had the umptint email offering me Plat and waive the $450 yearly fee and I no longer get the Cent. invite, since I have turned it down every time (no waiver of fee).
Pay your bills every month and charge a lot and every time they turn down your purchase, ask for a credit release...........that's how I got to $350K. Been as high at $500K but only needed it twice.