dress up like a security guard and knock forcefully on the back/exit/stage/emergency door. works great =)
or else, get a press pass in advance. don't forget your camera, notebook, recorder, and preferably assistant. that works like magic, too =)
just some suggestions.
Sorry you missed the LA Convention but I have a more exciting proposition for you. There will be a huge AAI/Atheist Foundation of Australia in Melbourne, Australia next year featuring Richard Dawkins, PZ Myers, Peter Singer, Dan Barker, Taslima Nasrin, AC Grayling and many more. Details at The Rise of Atheism Global Atheist Convention website.
If you've always wanted to see Australia - this is the time to come.