They make a big effort to assemble a wardrobe that seems effortless. Guys and girls alike go for tight jeans, flannel shirts, Buddy Holly glasses and vintage clothing. They sport a cooler-than-thou, I-could-care-less attitude, and they try hard to be ironic at all costs.
In your day, they might have been called indies or the artsy crowd. Before that, they were hippies. Today, it's not so much "peace and love," but rather an appreciation of independent music and a taste for fringe movements that defines them.
Don't try to outdo hipsters at being hip. You might remind them that there's often a fine line between hip and smug. Dismissing others because they aren't up on the latest is simply, well, not cool.