How can I pursuade a parent to let me get a new phone, even though I got it in August 2007?

My contract ends in August 2009.How can I pursuade a parent to let me get a new phone, even though I got it in August 2007?
What type of carrier do you have because with some carriers you can by phones before your contract ends. Then you can save money and buy the money your self or do chores.