Being cool is something everyone aspires to be, but no one seems to know what it is. Also: is aspiring to be cool consistent with being cool? It isn’t the same as being good or socially desirable or beautiful, though elements of those qualities permeate coolness. It is an odd mixture of the moral, the aesthetic, and the likeable. It is, as has been remarked, a somewhat mysterious quality, elusive, hard to pin down–though there is a good deal of consensus about who has it and (more obviously) who does not. For me it begins with the shoes (and always has): a cool person must wear cool shoes. Then come the pants (or the skirt): this too has to be cool, though I am not so stringent when it comes to leg wear. The shirt or jacket is also judged for coolness (I like only three-button jackets). Then the hair reverts to the feet: a cool hairdo is a must, though the scope for coolness here is limited by the facts of nature (baldness being the main enemy of hair cool). What else must be added? It is generally agreed that independence is central—not following the masses. The cool person has autonomy, detachment, a willingness to make up his or her own mind. This independence covers everything from politics to fashion. It includes taste and judgment—in clothes, music, art, reading matter, comedy, speech, and even posture. It need not involve rebelliousness, though it can involve that; indeed, stridency of any kind is alien to coolness. The cool individual always maintains ironic distance, a sense of humor, a certain playfulness, and a reluctance to get overexcited. He or she is always tolerant and broad-minded (except where the matter of shoes in concerned). Cool headedness is part of coolness—not “losing one’s cool”. A cool person keeps his cool, his amused detachment, and his principled lack of enthusiasm (in the old religious sense). He tends not to say much and he speaks carefully, sometimes inarticulately (James Dean comes to mind). The cool person may suffer but he is not voluble about it—though it may be conveyed by the look in his eyes. Suffering in silence is the mark of cool. The shoes already speak volumes.
An interesting aspect of the concept of cool is that things as well as persons can be cool. It is a question which of these is logically prior. A cool person wears cool clothes, but the clothes are not cool because the person wearing them is; and similarly for music, hairstyles, speech patterns, etc. Even in intellectual and moral matters the intrinsic coolness of the object is crucial: a cool person is one who favors cool ideas and cool values. Thus we can’t separate the coolness of a person from the coolness of her possessions, styles, and beliefs. If you want to be cool, you have to surround yourself with cool stuff (which may include no stuff at all). There are no cool people living in uncool houses, with uncool musical tastes, and uncool political opinions. For me choice of conversational topic is a sure marker of the cool and the uncool: someone who just mouths platitudes and repeats clichés is the epitome of the uncool, while someone who picks up on something unusual stands a fair chance of passing the coolness test. Of course, someone who is trying his hardest to be cool is not going to make the grade: one must not aim to be cool. Ingratiating oneself is also unlikely to qualify one as cool, though when done ironically it can do the trick; irony is always conducive to coolness. Naturally, writing about coolness, or claiming it for oneself, is death to coolness—but then again the cool person is happy to take a holiday from being cool. Not being cool can sometimes be part of being cool. You begin to see why coolness is so elusive, unpredictability being of the essence.
One of the big problems faced by coolness these days is its commercialization. In the good old days very few people were cool and the concept itself hardly existed, but the marketers latched onto coolness as a selling point long ago (sometime in the 1950s). Now we are programmed to be cool and desperate to get our share of it: not being cool has become a source of shame and insecurity. So coolness has lost its minority appeal: it no longer sets a person apart. New ways must be found to assert one’s coolness. Fashions, in particular, spread like wildfire, so sartorial coolness can only be maintained for short periods. I remember when bell-bottoms came in around 1964: I was one of the first to wear them in my hometown of Blackpool and was looked at askance by many a straight-panted passer-by. It was cool for a while, but before long your Dad was wearing bell-bottoms. Ditto long hair and side burns. It has now become almost impossible to be cool in one’s mode of dress, because the clothing manufacturers are onto new trends so quickly. Rap music was cool for about twenty minutes, but then it was swallowed up by the fashion-entertainment industrial complex. Capitalism and cool are not natural allies. I have noticed that biker gear has not been commercialized in this way, probably because not many people ride motorcycles, so it is possible to retain some measure of cool by adopting the biker look (even fat old guys can pull it off). But in general it is hard now to generate new forms of cool that have not been debased by capitalism: you really have to go out of your way to find pockets of cool that you can dip into. Accordingly, coolness has become rarefied and difficult to detect; it is too easily exploited for commercial gain. Still, it is good to keep trying to find new varieties of cool and adopt them. Cool has always been creative and now we need to be creative about cool itself. I have some ideas but I’m reluctant to share them, because of the aforementioned bastardization. 
 Clue: they involve playing with the ideas of diversity and inclusiveness, the latest rigid orthodoxy. I also think Minnetonka shoes are pretty cool viewed through an ironic lens that is not avowedly ironic. And Buddy Holly is cool again, especially his sentimental stuff.
“It’s better to be dead and cool, than alive and uncool.”
—Harley Davidson (Mickey Rourke) in the movie Harley Davidson and the Marlboro Man (1991)
I remember the first time I felt the distinction between being cool and uncool. I was fourteen and deep into the band, “KISS”. Posters on my bed-room walls and all that. When my urban sophisticate friends from Chicago came for a sleep-over, I caught them snickering at my posters. Their talk was of Led Zeppelin and Black Sabbath.
It’s funny that you mention that because I find Zep and Sab the epitome of uncool–completely fake cool
It’s uncool to try to be cool. Innate coolness. You either have it or you don’t.
None are so lofty or oblivious or conniving in their, “presentation of self”, as even to approach the ideal (the chimera) of a native sort of cool. Now, on the other hand, some of Geezer Butler’s bass lines in a couple of early Black Sabbath albums are as funky and cool as hell—they, at least, approach the ideal.