You know what, since I'm thinking about it anyways, let's talk formalwear accessories. Most of these are traditionally menswear but a bit of gender fuckery is good for the soul, and frankly most of these are about making your mass-produced clothing fit and lay properly without having to go to the tailor.
Shirt stays: these go around your thighs to hold your shirt down, so that it stays smooth and tucked in. They're usually elastic, with 1-3 clips, and if you wear skirts frequently this is a GREAT way to make sure your top doesn't ride up. The clips will be visible if you're wearing something tight, so loose pants or skirts are where these do best. There's also an insane version that clips to your socks, but that is for lunatics. If you wanted, you could also use one of these clips to hold up thigh-highs.
These do a great job of smoothing and narrowing the waist area by keeping your shirt from bunching there.
Sleeve garters: usually metal, leather, elastic, or silk. These are usually worn with button-down shirts to adjust where your cuff falls on the wrist or hand. They're properly worn on the upper arm, and you pull the fabric of the sleeve above the garter until you cuff is where you want it. Because this creates a puff of sleeve at the bicep, it also broadens the appearance of the shoulders. It's great if you're working with your hands or if your sleeves are often too long for your preference.
Waistband clip or belt adjustment clip/buttons
Three different ways of tightening the waistband of a pair of pants or a skirt. You're not going to get more than an inch or so tighter without weird bunching, and for most of these you'd want them to be hidden under a shirt or jacket, but they do the job if that's something you're having issues with.
Collar pins: There are so many fun ones out there, both with and without chains. They're not terribly practical, though the slight weight may help keep your collar where you want it. Also consider collar tips, which pin (surprise) to the very tips of your collar points.
Sweater clips/guards: meant to hold your sweater or cardigan mostly closed. Great if your cardigan doesn't button, or if you don't like it to be buttoned all the way.
There's tons of other stuff out there like this--etsy is a great place to find this stuff. A lot of these are old solutions to the very modern problem of mass-maufactured clothes not being as one-size-fits-all as advertised, but they're also a fun way to put a bit of personality into businesswear.
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