Most people don't think about updating their credit card until they've discovered they can't get into the site. Click "my subscriptions" in the "User Menu" on the right hand side. Then click the third tab that says "my invoices." Click "pay" next to the invoice. It will lead you to the place to update your credit card information. Note that you need to enter the whole thing, NOT just the new expiration date. Enter the whole number and the expiration date, click save, and your access will be restored as soon as the charge goes through.

If you're one of the exceptional people who thinks of updating their credit card BEFORE getting blocked out, instead of clicking the "invoice" tab, click the "bililng information" tab. There's you'll see the place to enter your new information. Please enter the whole thing, not just the new expiration date. Click Save, and your next invoice should go through without a hitch.