Dress codes at the workplace give helpful guidance as to what's expected, but they can also cause friction between coworkers. If you feel one or more of your coworkers isn't adhering to the dress code, tread carefully, because this is a sensitive subject.
Assess Your Concerns
List the reasons that your coworker's dress bothers you. Determine if you're concerned about fairness, because you feel you should all be subject to the same rules, or if there's a substantive business concern; for instance that the person is inappropriately dressed in front of clients or vendors.
Talk to a Manager
If you can, address the issue indirectly. Without identifying the coworker you're concerned about, talk to a manager about your general worries that the dress code isn't being adhered to. Suggest the manager reiterate the dress code to everyone in a memo or staff meeting. That way, no one needs to feel singled out, but the policy is highlighted and made clear.
Directly
If you or another person in the office have a close relationship with the colleague who's dressing inappropriately, try having a private word. It's best to do this away from the office, perhaps over coffee or lunch in a low-pressure setting. Be tactful about your concerns and express them in terms of the person's own self-interest. Tell her it will be easier for her to be taken seriously for promotion if she dresses smartly.
Go Shopping
A fun way to address this problem can be propose a lunchtime or after-work shopping trip. Go somewhere that's going to have a nice selection of business casual wear at affordable prices. Then make non-personal suggestions -- point out a fitted, button-down shirt in a solid color and say it would look great at the office, or a knee-length skirt and point out how flattering it would be. Some people need suggestions to see themselves in professional wear.