If you have done clips for the publication before, you might have a couple of those clips tucked into a notebook. If there is an event where press access is under stricter rules, don't be shy about nicely asking your editor to set it up the access for you. You may be put on a guest list of sorts, and your arrival will be expected by the person in charge. In such a situation, expect to show photo ID to prove you're the right person.
If you're freelancing without a media organization attached, or writing for a blog, go to Step Three!