WikiHow has a good article on How to Get out of a Cellular Service Contract. While most of the methods involve smooth-talking your way out of the contract, they do point out my favorite way to get out of a cellphone contract: Wait for them to change your contract terms. This actually happens quite frequently, and I’ve done it successfully myself. In my case they raised the late payment fee from $25 to $39 or something like that. They’ll bury this in your statement in tiny print, sometimes on the back of page 6 of 7 or something.
But the law says if they change your contract, you can either accept or cancel within 30 days. Cancel! This is perfectly fair as they are changing the rules on you! Note that they may cancel you that very second, so be prepared. In fact, I remember my conversation with Verizon Wireless like it was yesterday:
Me: Hi, I’d like to cancel my contract.
Verizon CSR: That’ll be a $300 cancellation fee, sucker. (okay, she didn’t say “suckerâ€)
Me: It says with my statement that you are changing my contract so I can cancel without any fees.
Verizon CSR: Let me check. [pause] Yes, we raised the late payment fee. Have you ever had a late payment?
Me: Umm… maybe…
Verizon CSR: Let me check. [pause] No sir, you have never made a late payment. This change will not even affect you. Do you really think it’s fair that you are breaking your contract for something that doesn’t even affect you?
Me: Hey, I wasn’t the one who wanted to change the rules.
Verizon CSR: But you never pay late.
Me: Please cancel my contract without a fee as the law requires.
Verizon CSR: But blah blah you’re a bad person blah blah may god have mercy on your soul blah blah
Me: Please cancel my contract without a fee as the law requires.
Verizon CSR: Okay your account is cancelled as of immediately. Your phone will no longer work. Your number is gone. Your last bill will be prorated. Goodbye. [click]
And true to her word, my cell phone was dead that very second. No problem, I put it on eBay and got me a better deal elsewhere 
Some of the other tricks seems more like long shots. Here are the two I think are the most likely to work:
Get off the grid. Study your provider’s coverage map and find a town (maybe in Alaska?) with absolutely no service. Then tell the company you’re moving there. They’re not legally required to cut you loose, but frustrated consumers have reported success.
Get a lemon. Get a known problematic phone, complain 3 times, be let out of a contract due to your local lemon law.