Attempt to cancel. And sound like you mean it. But be polite!
I'm serious. If you're a wireless customer in Canada, who is on a contract (that's most of us), then you're overpaying, unless you go through the motions of attempting to cancel your plan every once in a while. It's probably worth your time to call once per year. There's always a better deal available, and if there isn't, tell them you want to think about it and then just don't call back to finalize the cancellation. I would say most people on smart phone plans can save $20-25 per month if they are willing to invest about 1 hour per year in going through this process. The same principles apply to your cable and internet providers. In fact, they're probably all the same company.
Have your story figured out in advance. Either you are coming to the end of your contract and you're thinking of switching, or you've got a better offer from another carrier or something like that. I personally like leveraging another carrier. Especially a discount carrier. Tell your carrier that you're getting offered a plan for $45 per month for unlimited EVERYTHING and you just can't pass up that deal.
Call your wireless carrier (*611) and choose the corresponding number option to cancel. This will get you to the retentions department. They likely call it "Customer Relations" or something fluffy like that. This skips you past the first line of customer service people who have little power to give you a better deal. The retentions department has much more power to offer you deals to keep you as a customer.
Remember. Be polite. The people on the phone are just doing their jobs and if you're reasonable, rational and pleasant, you'll get a better deal. Only in extreme circumstances should you pull out your best irate "the customer is always right" routine. That won't get you too far, unless you are actually arguing over a major mistake the carrier has made, like a big billing error, which they usually try not to admit to anyway.
Write down what they are offering you. Have a pen, paper and calculator ready to see what the potential offers will save you. Or, better yet, have a spreadsheet going. I once had a poor "Customer Relations" rep on the phone for 2 hours one night going through all my options (minutes, long distance, data, SMS, MMS, voicemail, etc.) to try to justify staying with my them. Certainly, he couldn't match the $45 per month I was quoting from the discount carrier, but he was trying really hard. He got as close as he could and then I conceded that by staying with them, I was saving several hundred dollars in cancellation fees, so "he'd won me back!". I saved almost $40 per month on my bill and he felt like he'd done an awesome job. He really had.
Save the details. Whether you're using a spreadsheet or pen and paper, make sure to save the offer you were promised and also write down the confirmation number that they should give you. Make sure to get that confirmation number. I've had a few instances where the offer I was promised wasn't "processed correctly" and have had to call back to get things fixed. Without the proper details, you might have to start all over again. That's not fun.
Pay attention to your bills, especially the one right after you've switched to a new plan. I've definitely had challenges with incorrect billing, and I even suspect that when you switch plans in the middle of a billing cycle, the major carriers try to use that scenario to confuse you with a bunch of "partial charges" that you expect to even out the following month, but they never do. They count on you being either still confused, or too lazy to call back. The last plan change I made a few months back, resulted in a $70 overcharge in the form of "partial charges", about which I had to call back and argue. I had the math all done and I still couldn't convince 3 different people on the phone that they had made a mistake. All they could respond with was a generic "This is how partial charges work. It will all even out". To which, I asked "So, I'll see a $70 credit next month?" and they answered "No". In the end, they gave me the $70 back in the form of a "goodwill credit" as a way of avoiding having to admit they made a mistake, or worse, tried to rip me off. In hindsight, I think the better route to go is to make any changes to your account effective only at the start of your next billing cycle. As tempting as it might be to try to take advantage of your new hard fought deal immediately, waiting a week or two until your next billing cycle starts, might just save you some hassle. Otherwise you could end up wasting another hour or more on the phone.
Good luck and happy hunting for good deals!