In college they always told us to wear a suit for interviews, so I was a good little student and did just that... and felt ridiculously out of place when in my interview for my internship I was in a suit and black leather pumps and one of my interviewers was wearing cargo pants and a tank and the other one was wearing shorts and a Hawaiian shirt. Since then, for interviews, I go with a nice button-down top and tailored pants, and flats (I'm 6'1, so shaking hands with a 5'3 interviewer is already awkward without the heels.)
For rehearsals, I generally wear nice jeans, flats or nice looking tennis shoes, and a dressy-casual top if it's a weekday, or a good looking t-shirt if it's a Saturday rehearsal. I also always make sure that I look decently turned out with hair/makeup-- I once had an ASM who used to come to rehearsal with her hair still wet just thrown up in a ponytail on Saturdays... which I think sends totally the wrong message. I always look in the mirror and make sure I look like I've been sitting staring at the script or something for 3 hours before the rehearsal-- to admit to the cast that you were asleep an hour ago just feels disrespectful to me, I'd be mad at any actor who showed up like that.
For performances, I always go with nice looking black stretch jeans (I buy a new pair every show or two at walmart-- keeps them from getting faded), black belt, black tennis shoes, a black tee or tank top, and a long sleeve black men's dress shirt. I always get the dress shirt one size up from what I'd normally wear-- it's not so much bigger that it looks baggy, but it gives me freedom of movement. The booth where I spend most of my time during shows isn't very well air-conditioned, either, so having a loose thin material is better than wearing something that won't let you breath!
For opening nights, I take the time to blowdry my hair, wear studs and a necklace, and sometimes change out the tennis shoes for black flats and the jeans for trousers, if there's no set changes or anything that require me to do major physical work every night.