I want a new cell phone...AT&T?

I want a new phone that is $349.99. It is so much because I just locked myself into a plan about five months ago. How should I handle bargaining with AT&T over the phone. I thought I would threaten cancelling my service being that I get 20% off of verizon and sprint because of my job. How would you ration with the customer service reps? I feel as if I will need to speak to one of their supervisors right away. Any tips?