Sometimes, cancelling a service that you don't want or need anymore can be a giant pain in the behind. However, calling outside of regular business hours might turn the tide your way:
The biggest tip is to call well outside of normal business hours -- in my company, customer service was open 24/7, but the retention department closed in the evening. If you call, say, before bed, or during the middle of the night, you'll just be talking to a regular CS rep who has no incentive whatsoever to keep you as a customer. It can turn a twenty minute phone call into a two minute phone call.
If you've ever had to deal with pushy reps this could be a lifesaver. What's your best tip for getting out of services you don't want? Thoughts in the comments.