How can i convince my parents to let me get a horse?

Im nearly 14 years old and i am begging my parents to let me lease.. My mom is getting a new job soon so we will have more money soon. I have spent all my summers at the barn and all the cold hard winters but my parents still always say no. i stopped bugging them about it but when i ask them about it every now and then they say we cant afford it. but i know in late august we can. I am going to try to take care of my dogs to show them responsibility. and i have saved almost $300 to help. what else can i say or do? what did you do if this happened to you?