How to i make my thick wavy hair straight when it's long?

I have a lot of really thick hair and so I have to keep it short (chin-length) and flat iron it every day.When my hair is longer, even when I flat iron it, it's so thick that it will begin to curl and gets really wavy. I'd really like to grow my hair out, but what can I do to make it straight? ( i'd actually like to stop using my flatiron)

I have thick wavy/curly hair. My hair is just past shoulder length.

For me, having layers is the easiest way to manage my hair and keep it straighter.

I either flat iron or let air dry while hair is in ponytails - both make my hair fairly straight.

if i were you i would get it chemically relaxed. most salons offer this service and it commonly costs about the same as a perm would. it causes some damage but beings that you have such thick hair it shouldnt be too much of a problem. it is honestly the only way that you can keep your hair straight without using your flat iron every day. 

You have to oil your hair. Try this at night, in hot oil mix lemon juice and apply it and rinse off in next morning. Do it at least every week and you will get straight hair without using any chemicals.