At some point, you're going to start dressing like a director. Black T-shirt, jeans, sneakers, baseball cap.
Most directors put on the uniform because they want to look like what they hope to be taken seriously as. And it's not until years later that they discover the purpose behind what they wear. If ever.
Here's why you wear what I wear that they wear.
Baseball cap: If you're shooting exteriors, a baseball cap keeps out the sun. Unfortunately, the bill of a baseball cap isn't exactly well-designed for looking through the viewfinder. The reality? We're losing our hair.
Jeans: Jeans are relatively cheap and durable and aren't ruined by stains. As glamorous as directing sounds, I've spent more time shooting in garbage dumps than in the catwalks of Milan. And no, you don't get to ride into the set in a sedan chair, ordering your minions to find a shot. If you're not on your hands and knees with a viewfinder, you're not directing.
Sneakers: There's a famous story that's been attributed to Steven Spielberg, although it may be apocryphal. An aspiring director asked him for advice and he answered, "Wear comfortable shoes." Sounds glib, but it's actually great advice. You're on your feet constantly. Good directors don't bark commands through a megaphone. They generally station themselves next to the DP in order to work with them on the shot, then move into the set to adjust the actors' performances. On a long lens, you're moving hundreds of yards between every take. (I shot a job on location in Namibia, and since the schedule was tight--and I like to keep the energy up--I found myself running 200 yards between the camera and the actors between each take. Over rocky ground. Try that in a pair of Gucci loafers.)
Black T-Shirt: This is the most telling bit of wardrobe, and there are a bunch of explanations put forward for it. 1) Black is a slimming color. There's probably a lot of truth to this, especially if you don't run around like I do on a set. The craft services table is usually brimming with all sorts of candy and cookies. 2) Black doesn't reflect. There's probably a lot of truth to this one, too, especially on car shoots and sets with lots of reflective surfaces. But while black makes you reflect less, it doesn't make you invisible. Even in a black shirt, your reflection is going to be seen. 3) Black goes with everything. Bingo, but probably not for the reasons you think. It's not about fashion, it's about packing. My work requires me to travel--I once had to fly to Zurich for a single meeting--and in order to travel I have to pack. I don't like checking luggage, so I try to condense my clothes as much as possible. A couple of black T-shirts and a couple pair of jeans and I can be presentable (albeit predictable) for a week.
Besides, I look like a director. Which helps when I'm meeting people I need to impress with my authority for the first time.