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I've learned a bit since then. While it is still possible to use iTunes on the computer, you can also manage your subscriptions from the iPad. Though they don't exactly make it obvious.
It is done by accessing your Apple ID account through the App Store. Tap your Apple ID. It appears at the bottom of most of the tabs in the App Store. Choose View Apple ID, enter your account information when prompted, then select Manage under Subscriptions.
Only magazines that are both paid subscriptions and in the Newsstand can be managed this way. Magazines that are not in the Newsstand will have their own methods, which you should be able to read about in the App Store descriptions or on their support site.
Free magazines; well, they're free. No need to bother unsubscribing. Just delete the app.
Also, you can only prevent automatic renewals. You can't actually cancel an already paid for subscription period. Just make sure your renewal is canceled. After that you can delete the app if you don't want to bother with the remaining issues.