Look like you care about how you look.
"Carelessness seems to signal that you don't respect your coworkers or yourself," says Sylvia Ann Hewlett, author of "Executive Presence." "You certainly don't respect the client if you show up with soup on your tie or bitten nails, anything to make you look unkempt."
Tailor your look to the situation.
"If it's Microsoft, it's one outfit; in the US Army, it's another," says longtime executive recruiter Russell Reynolds. "You have to look like you belong to the group, [and] you have to look a little better than the group." To do that, Hewlett says to "pattern yourself" after someone who's particularly good at dressing a bit better than the rest of the group.
Stay away from the suggestive.
"For women, a major blunder is sexually provocative clothing," Hewlett says. "It's tremendously undermining of your gravitas, because it's either distracting or threatening. No matter your projection of your capabilities, if your skirt is too short or your neckline too plunging, you get struck off the list for a big client meeting or a promotion."