How can i get my hair to stay straighter for a long time?

Im german and black and my hair is more on the curly and soft side but when i straighten my hair it only last for like a day or two. I dont press my hair or anyyhing like that. Just wash condition, leavin straightening conditioner and heat protector. Anything missing??? Suggestions? Oh anf i dont blowdry. Air dry and then i flatiron. Pressing my hair makes it all frozen and retartit