Ok, for starters this post has been done likesooo may times already. And the same answers apply.
1-by a J or F class ticket!
2- wear your best "Armani" suite and have the top tier Frequant flyer program card (gold or platinum usualy does it)
3- And this is the last resort! Shower the check in agent with gifts, compliments and BEG, and if all else fails turn left once on the plane and act normal!, like you belong here. Alos hope to hell that no-one is booked into the saet that you've taken!
Hope this helps!!!