I get asked on a somewhat regular basis (usually by interns or kids just starting out in theatre) where I buy my black clothes for running a show. My first response it to usually say that you should wear whatever is comfortable, but I’ve worked with a few people who wear things that are not practical.
I normally wear Lululemon (yes, the yoga clothes) Studio Pants, a black shirt (I get mine from the Gap because they fit me nicely and don’t wear thin too quickly) and a Lululemon (my friend got me hooked a few years ago) Define Jacket on top. I like the pants because I don’t get overheated on a busy show and they allow me to move around without feeling constricted. The jacket is great because it’s lightweight but keeps me warm enough in the icebox we like to call a theatre. It’s also tight around my body, so I won’t get caught on anything. (Flowy tops are not a good idea for backstage, especially if there is automation involved. I’ll let you imagine what that could look like on your own.)
Shoes are really important. Any woman will tell you that, but having something comfortable backstage is key. You’re going to be standing on what is probably a hard wood or concrete floor for hours at a time. Chucks and Toms may look cute, but your knees will hate you. (Even if they don’t now, they will in a few years.) I have a pair of Nike Free’s that have been great. I didn’t know much about them, but I went to the store and told them that I needed all black shoes that I could stand in for ten hours. They’re great and I love them, and I don’t want to chop my knees off after a ten out of twelve.
Again, getting something comfortable is what is important. But if you plan on making a career of working in theatre; investing in higher end clothes will save you money in the long run.