I'd like to open up this topic for comments. As a starter:
* Always say "I can't remember" instead of "I don't know"... chances are, you probably really don't remember, and forgetfulness is usually viewed more favorably than ignorance.
* Dress appropriately for the situation you're in... Hey, I hate to dress nicely as much as (perhaps more than) the next guy -- but the truth is that people evaluate you by how you dress. If you're in charge or presenting, dress one step nicer than the attendees.
* Carry a newspaper with you on the way into work, preferably The New York Times or Wall Street Journal. If possible, glance at the front page summary while walking in, and then leave the paper open on your desk during the workday. Sure, you can get your news for free online, but not everyone knows that. People associate reading the news and being up on current events with intelligence.