Request access in advance either by phone or by mail. Put written requests on company letterhead. Explain why you wish to cover the event and any special requirements you have, such as equipment or crew members that you will be bringing.
Show up to the event promptly. Make sure the press pass is visible. Be ready for a security check.
Introduce yourself to marketing and press coordinators. Ask to be included on future e-mail lists to receive notification of upcoming events.
If you need to try to gain access to an event at the last minute, try showing up to the venue and asking to speak with the press coordinator. It's possible that showing your credentials and explaining your need for access could get you in. But it may not work for more secure events.