Find any contact information for the magazine. This can usually be found either in the magazine's table of contents or on their editor's page, both at the beginning of the magazine. Usually they will list a phone number, email address and mailing address.
Contact the magazine by phone first and take note of whom you speak to and when. Tell them you would like to cancel your subscription and be firm about it, but not rude. Usually if you are firm and forward you will be able to cancel your subscription.
If you can't get hold of someone over the phone, check the Frequently Asked Questions (FAQ) section of the magazine's website, assuming it has one. Look for a question about how to cancel a subscription with that specific magazine, and follow directions accordingly.
Send an email to the magazine stating that you have spoken with someone from the magazine, the date on which you spoke with them, the result, and that you want to confirm that your subscription is canceled.
Write a letter that is similar to your email. Let them know by all means possible that you wish to cancel your subscription. Oftentimes magazines ignore the first cancellation attempt, or keep sending magazines for a few extra months. Contacting them multiple times shows that you're very serious and, should you keep being charged, you will have your three contact attempts on file to prove they were notified.