Suits
In old probably-fabricated memories from my childhood I can hear this advice from my dad: son, you ought to avoid taking jobs that require a uniform.
This advice has stirred in my mind for the last two decades and is on the cusp of becoming a manifesto about identity. It informs my clothing choices at the very least, and it has certainly has an effect on the shape of TMAP.
It's easy to pick a look and wear it every day. There are many people with similar beliefs and personalities to yours. The internet will show you thousands of options, and they can each bend slightly further in your direction. If you're lucky, you'll effectively be wearing your own clothes. Besides, you'll save the people you meet a lot of time when they can quickly drape you in shared cultural associations. But after three days, the outfit wears you, so you'd better choose wisely.
Being a professional is not about wearing a pinstripe suit any more than it is about wearing a heavy cotton duck work coat, or ordering people around, or having an MBA. Being a professional is about taking what you do seriously, and being confident in the knowledge that when you do it, it's worth money.
We wear outfits to suit our environment. At a wedding you wear a suit because that's what's done. Under a truck you wear black everything to hide the grease stains. At a cocktail party you get a little weird.
Knowing where you are and what the context demands of your behavior; knowing what's expected of you and how to do it: these are the marks of professionalism. It's how you know you're the one wearing the suit and not the other way around.