Of Practical Matters
I’m quite sure everyone here has managed to stumble across an anachronism so horrific it leaves you wondering what kind of idiot would write such tosh. It’s like the Regency lady being divested of her bra and panties (yes, I have seen this. I promptly tried to eliminate the memory via a large quantity of brain bleach).
Frankly, it’s more than a little bit important that any kind of SF or fantasy that’s not using present day as its time period gets the clothes at least partly right. After all, who would want to see Space Pirate Dashing Hero saving the universe while wearing a farthingale? Unless he’s into period crossdressing or some kind of bizarre bet went wrong, of course.
Hell, you can sink your historical by neglecting to have cotton as a highly prized, expensive fabric in the years before the American south got into cotton farming in a big way. Just as in the early parts of the 20th century, asbestos paneling was the expensive, prestigious way to cover your walls and ceilings.
For futuristic purposes inventing new synthetic materials with names that sound like the effect you’re after can work, as long as you don’t overdo it and start mentioning things like “glassite” that just happens to look and act exactly like plain old glass (I’m not going near nylonite and don’t even think about kryptonite unless you want a nasty lawsuit).
As for matters of what people will wear, there’s always going to be a combination of a few factors: what’s available in your characters price range, what’s comfortable and practical, and what’s fashionable. The latter will affect what can be bought when, the former what can be bought, period, and the middle what the character actually wants to buy.
Clothing repair matters too – it’s… challenging to expect your characters to go through massive trials and have their clothes remain intact and undamaged. Let’s face it, modern clothing lasts a few years before it starts to fall apart. All fabrics rot at different speeds, with different results. By the time your character has invaded hell and fought his way to the infernal throne, he’s going to be down to maybe one change of clothes that’s half-decent or he’s scavenging from the demons he’s taken down along the way.
Which leads to the… ahem… underpinnings of the whole business, namely underwear. Getting the right names to the assorted unmentionables matters, especially in something historical (and a lot of fantasy – you throw modern undies into your Tolkienian adventure and you will lose readers. Unless you’ve got a damn solid, properly Heinleined reason to have modern undies sharing a world with swords and sorcery and medievalish mayhem).
There’s also the reason for certain undergarments. Women have often provided themselves with a certain amount of buttressing to prevent excessive bounce. This has become a kind of fashion item, leading to a rich and fascinating history of corsetry (and I can say from experience that a well-fitted corset is a lot more comfortable than a bra. The weight is carried by the core body instead of the shoulders). Men have, as a rule, opted for underwear that allows them to do what they need when they need it, with the minimal amount of fuss. If they wore any in the first place, which is not guaranteed (the same with the women, I might add. As little as 200 years ago, a middle class person did well to own three sets of clothes: the Sunday best, and two every day outfits. That included the small clothes (aka undies).
Your future epic could well need interesting attachments to deal with issues like needing to wear environment suits or spacesuits for days on end. And possibly a whole range of recycling plumbing that minimizes the amount of waste that gets out. Kind of like the camelback on steroids. And maybe you need to deal with issues caused by having to repair the things. Something that protects against unbreathable air is not going to take well to a needle and thread.
All of this matters – but it doesn’t need to be laid out in excruciating detail (that’s what the Heinleining is for). It just has to be there so that when your Love Interest’s chemise is ripped during her escape from the Antagonist, it’s pretty clear that Antagonist either caught her when she was in a state of partial undress, or caused said state of partial undress. And of course, so she’s not wearing her chemise over ceramite body armor. Because that will get your book walled as soon as I stop laughing hysterically.