Have some patience, show you are responsible, make sure you can afford it.
If you have curly hair your parents might not want you to straighten your hair because they are afraid you might damage it so you need to understand, do not annoy your parents about it. Make it seem that you are mature about the current situation, show them the positive reasons of having straight hair , do not talk about the consequences of straightening your hair.
It is better to let it air dry then straighten it, less damage to your hair
try talking to them and resaning with them, don't make a trajic change at first.
Honestly, you can't convince them. Either they will let you or they won't.
if your parents wont let you get sponsored, well, id yell at em.
you say parents are going with
go on "strike" until they say yes, and get their "yes" on recording. do not loose the recording, and do not refuse to eat or drink
Convince them by actually shaving it off and say "hey mom,hey dad this is the new look I'm going for,it's called "I make my own decisions" do ya like it"? And then ask their opinion on pubic hair
Be on your best behavior ?
say please
you ask them
make a deal with them and stick to it :)
tell them you want rats