The unfortunate reality of dress down is that there seem to be more don'ts than dos. The good news is that most of them are just plain common sense.
Don't ever underestimate the impact your appearance has on the way you are perceived (particularly from seniors) and therefore your career progression. If you look like a slob people are more likely to think you are one. Whether or not it is true. Sloppily dressed signals sloppy work, lazy, unreliable and unprofessional. The flipside of this obviously being that if you look smart you will be more likely to come across as sharp, professional, assertive and on the ball.
Don't get uppity about it and think because you haven't seen a written policy that there isn't one lurking in HR which might be dragged out to give you a real 'dressing down'. 'No-one told me I couldn't wear short shorts and a mesh tank top' will not help further your career. Trust us. A written warning for a serious lack of style on your personnel file is an unnecessary evil!
If you are male, don't wear a tie with jeans or combats in an attempt to dress down but not too far down. This is a surefire way to demonstrate your lack of dress down savvy.
If you are female, clingy gold lame, sparkly stilettos, disco nails and glitter make-up are all out. Even if you set your alarm at 5.00am to perfect the look no one will believe you have not just arrived hot off the dance floor.
The biggest don't is pretty simple. Don't ignore company dress policy in an attempt to stand out from the crowd. Great, you're an individual. You don't want to be the infamous individual who got the dress down policy revoked for everyone else.