I define "hipsters" as poseurs who are immitating people who are actually hip, or at least who they perceive to be hip. A hip person typically has some degree of fashion sense, and a good example of the difference between this and a "hipster" is how a hip person might buy articles of clothing from the Salvation Army because they're cheap and/or unique, whereas a "hipster" would shop at some sort of boutique that sells exclusively fashionable vintage clothing and pay 10 times as much for more or less the same items.
A hip person is cognitive of what styles are fashionable, and finds a style that suits his or her self, but doesn't necessarily have to be wearing exclusively non-mainstream vintage or designer articles. A hipster either copies a hip person, the other hipsters he knows, or whatever he sees in "Vice" magazine. A few fashion items are always acceptible regardless of how much of a poseur you are or aren't: Chuck Taylors, black hoodies, tight pants, etc.
A hip person might look hung over or like they just got out of bed, but if they do, it's because they really are hung over and just got out of bed. A "hipster" might spend considerable effort faking that look by messing up their hair and intentionally wearing wrinkled clothes. A telltale sign of a hipster douchebag is that he or she is clearly "trying" to be fashionable. They often overaccessorize with some sort of scarf, hat, or outlandish shoes, and put a great deal of effort into making sure everything matches.
A hip person most likely is an avid listener of music, but doesn't particularly care how others judge his or her taste. He or she might listen to anything, sometimes including things that were once, or even are currently, mainstream. A "hipster" contrives his or her music collection to be seen by others as cool and "indie". They love vinyl records. Crappy music seems cool to them as long as they listen to it on vinyl, and especially if it's some sort of rare recording that they can be sure no one else they know owns. If you asked a hipster whether he cared if people thought his music collection was cool, he would say "no", but he would be lying. They care.
In fact, the defining trait that seperates legitimate cool people from hipster douchebags is that truly hip people genuinely don't care what other people think of them, or maybe acknowledge it a little bit but don't base their whole persona around it. Hipsters are obsessed with this fact, and build their entire personalities in an effort to make it look like they also don't care. They want, just as much as normal people, to fit in. A really hip person just naturally fits in because he or she has a likable personality and is fun to be around. All hipsters hate other hipsters and consider them to be phonies, and no hipster will ever admit to being one even if they fit all of the stereotypes.
True story: I was recently at a thanksgiving party where a lot of food was being served. The skinny, pale, threadbare sweater and tight pants-clad hipster-looking guy who had been putting on music all evening walked into the room. People weren't talking much at the time because their mouths were full, and hipster douchemaster sweater guy decided the room wasn't sociable enough for him, said "This room sucks", turned, and walked away. He could have just walked away, but he felt he had to verbally pass judgment on the room's occupants to make sure he seemed superior to them. What a dick.