To an executive, this is how you look Monday through Thursday, milling about with your peers:

To an executive, this is how you look on Friday, dressed awesome when the schlubs around you are looking “casual”:

You’re welcome.
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Author Update in 2024:
This was originally written in 2012. The pandemic changed a lot of things. My new suggestion is – just dress better than everyone else all the time.
This may seem like weird advice when the whole world is dressing down. People care a lot less about their appearance than a decade ago – and that is wonderful. Fully remote workers don’t dress up at all. You could and should see this as a benefit of modern life. However, I challenge you to see this as an opportunity. Human brains are easily tricked. Even when you know down to your very core that how someone dresses is not an indication of their intelligence or quality – your dumb brain can’t help but connect those dots in the background. Even crazier – if you work to look a bit nicer, your own brain will trick you into feeling better and more confident. There is almost no downside to looking better (outside of effort) and there is a lot of upside. Your choice.