how do I keep my hair straight?

liv.geisel
So I have REALLY thick hair, (and I talking really thick like I have enough hair for 6 people on my head) it's heavy and long and usually I just wear it natural bc it takes too long to do anything to it. I'm going to start flat ironing my hair again, but every time I do my hair never stays straight. It always ki inks back to its natural form, and frizzes up even more than usual. How you y'all keep your hair straight after flat ironing? I need all the secrets😂