Heavy metal has grown to be something of a religion and lifestyle, rather than just a musical genre. Metal is arguably the biggest movement in underground music and has been so for half a century. Metal won't die and you can easily count yourself amongst its disciples if you follow this easy fashion guide. Anyone can be metal and for many there's nothing cooler than heavy metal music, to which the fashion always comes second. I love heavy metal music and I wear a cashmere cardigan.
Band shirts are an absolute must. It doesn't matter whether you're into classical, proto-metal bands like Black Sabbath, Led Zeppelin or AC/DC or whatever, or the darker stuff, you simply must wear a band shirt. Black is the colour of metal and, as such, most metal band tshirts are only available in that colour. Typical metal iconography includes skulls, dragons, barbarians, barbarian babes, motorcycles and fire. Lots of fire. A leather jacket works too.
Not as important as the top half, but if you're sporting a leather jacket then I couldn't recommend getting a pair of leather trousers any more. Leather and leather goes so well together it's untrue. It's an unholy alliance! Jeans are also good, depending on what style of metal you're into. Thrash and glam fans will want tight, ripped jeans. For everyone else, just ripped ones will do.
New rocks are pretty cool, but maybe a bit goth, so go for some plain, black boots. The kind you can imagine fascist boot-boys wearing. Thick and heavy with steel toecaps to keep your tootsies from being trampled underfoot in those moshpits.
Festival bands! Tons of them, on both arms, see how many you can keep on them. I've got some that are six years old and they still look as good as the day I got them. They keep for ages if you shower with the right kind of shampoo and conditioner.
Oh, I should point out that the hair is the most important thing. Keep it long, but for god's sake, keep it neat. I hate have my face whipped at gigs by some grimy little metalhead's unkempt dreds. You can also wear long hair with lots of other fashions too, plus it looks great with a cashmere scarf!