I think there should be a law that if you're not an actual fan you're forbidden to wear them.
me too, but sadly my actual favourite band (which is very identifying, but it's a metal band that was biggest in the 80s) is hard to come by on a t-shirt. Sometimes I wish I was a big Ramones or G'n'R fan just for the readily available shirts!
Things I think make a person look cool:
Not necessarily super tall or thin, but walk tall, with confident body language
If you're white, being a bit brown (I haven't got a hope of achieving that and won't even try, but I have to admit I do think it looks cool)
Healthy looking hair, preferably not totally straight unless it's quite thick. A messy bun usually looks cool. if your hair's a bit sunbleached that can look cool too.
The right jeans! There's a right fit out there for everyone, I just haven't found mine yet.
Rolling up your jeans not too perfectly when it's hot. Wide linen trousers are quite cool too.
Wearing cotton, or something that looks like cotton. Shiny is never cool.
A leather jacket that fits well and doesn't look too shinily new.
Not looking sweaty (unless you're running) - again I haven't managed to crack that one yet.
Unfussy accessories - a beaten up old leather bag, chunky silver rings, that sort of thing. Maybe a unisex straw hat or a trilby, if you look like you're wearing it because the weather demands it, and not just because you think it looks cool!
Most stripy tops look cool on most people - but again, natural fibres and loose is better than tight.
Converses, Superga or similar. Beat up old boots in winter. If you need heels, really plain classic black courts are probably the best.
Personally I never feel cool exactly in a skirt or a dress, although I might feel stylish or well put together when I get it right. Some other people look cool in them though. I'm not sure what the magic ingredient is.
But I also know people who look cool in all the "wrong" things so I think it's true to some extent that some people are just born cool.