Prove to them that you are responsible. Do all your school work and get the best grades that you can. Help around the house. Or ask for an allowance - $10 or $5 a week for doing chores. it is a small amount but it will most likely add up and you can buy it yourself :)
Show them the useful features and demonstrate some of the apps they can buy and their usefulness. After they get one, it will be difficult for them not to be fighting over it, so they might as well get two right off the bat.
Do nice things for them such as chores. Also, be really nice and appreciate everything small they do for you. Be responsible with the phone you currently own. Your parents will eventually come around to the idea of buying you the IPhone. Good luck.