Firstly, you need to suit up. This is fashion, not function. But don't overdo it, at least not if you're goal is backstage. You're going to need to get there 2-3 hours early (that's when the models, make-up, hair, etc. arrive). I suggest lingering about 20 meters from the back entrance, and cherry picking one of the unassuming models or stylists who have come outside for a cigarette. You give them a light yes I brought my lighter, strike up a conversation about esoteric medieval poetry or the latest issue of British Vogue, and then casually walk back in with them past the security guard. Of course this is much easier if you have a friend whos actually in the show, but a civilized chat with a Bulgarian waif should do the trick. Once you're in, just mill about not bothering anybody, until it gets crowded enough to blend in. If anyone asks you who you are, you respond with either "I'm Svetlana's agent", or "I'm one of the dressers and Herve told me to wait here", or the old standby "My girlfriend's in the show" (this last one will work for both men and women it's fashion, come on). If youre unlucky outside and have to approach the security guard on your own, well that's a tougher climb, but still doable. That's when I suggest breaking out the business card you got from the model agent outside (yes you had to smoke 10 filthy Gauloises cigarettes but he gave you his card after you lied and told him you owned an ad agency in Vancouver), claiming his identity, and sliding elegantly past the guard with the false purpose that only a fashionista can feign.
The easiest approach is to follow the instructions above, then sneak your way into the audience from behind (ouch). However, if you insist on breaking into the Balenciaga show with dignity eg: through the front door, then you have to prepare. First order of business is getting the costume together. You're probably thinking something expensive and chic...you're wrong. Dress horribly. Tight fitting vintage t-shirt with 20-year-old food stains, grey acid-washed jeans that fit terribly. And go heavy on the accessories. Pink shoes, vintage Dior shades, a baseball hat with the mascot of an unknown South American Jai Lai team on it, you know, dress "ironically". Digital watch. An oversized necklace with a Japanese Manga character on it - In gold. A belt with a stuffed Python for a buckle. This ridiculous garb will confuse the door folks, making them think youre some underground editor or soon to be celebrity. Now, even though you look ridiculous, pick up your phone and call a friend. Then start barking absurd things into the phone as you walk past the door people. You know, stuff like "Lindsey is such a slut I can't believe she's with Stavros... ugh I'm so glad I'm not in LA right now", or "I'm going into a show right now I can't talk but yes I'll put you on the list" then tell the door people that Russell Simmons is coming can you please add him.
Whether its front row or backstage the most important thing to remember whilst busting into a fashion show is that you have to believe your own bullshit. If you are "a photographer and you left your camera inside during the Betsey Johnson show and you need to get back in" then you have to act and feel the desperate indignation that one would feel during such a catwalk catastrophe. Be the lie. Otherwise, you'll be outside in the February cold stinking of cheap cigarettes and rejection. And you don't even smoke.