Seriously?? How do you convince your parents to buy you a iPhone??? This isn't that hard to do really. Here's what you've got to do.... get yourself a job and fork over $400 to your parents and have them drop by the nearest cell phone shop on their way home from work one day. I'm sure after they see the initiative that you've put into buying the phone, they will be happy to take the 10 minutes to pick it up for you. Then you can keep working and give them money for your monthly bills. Doing this you will have a iPhone in no time, and a sense of responsibility as well.
Other than telling you to maybe get a job, nobody here can tel you how to convince them, as we have no idea what kind of relationship you have w/your parents, anything about your parents or their monthly budget,.
Get a job and show them that you are willing to help pay for it. The iPhone is expensive and the service is somewhat pricey especially nowadays when money is very tight.
One factor to convincing my parents to let me buy an iPhone was that I had spent over $400.00 on applications for my old iPod Touch that was in rough shape and ready to break. (If you don't have an iPod Touch, or haven't spent very much on apps don't bother reading on.)I told them that if I didn't buy the iPhone, I would have to buy the other cellphone plus another iPod touch or else those apps would go to waste.