1. Ignoring the office culture. Culture is the invisible force that guides the thousands of things that happen in any office each day, down to how phones are answered or how meetings are run. By observing how others in the office act, you'll pick up on some very powerful messages about "how we do things here." For instance, do people modulate their voices when others are on the phone? Are they compulsively on time for meetings? There are lots of little things like this that will help you appear professional by simply observing and mirroring what you see.
2. Not taking notes. Yes, you really do need to remember the specific instructions you were given, including nuances, and not just the overarching idea—and for most people that means taking notes. And while a good manager is happy to answer questions, she won't be if the questions are ones she already answered when you weren't bothering to pay attention.
3. Not realizing that mistakes matter. In school, if you made a mistake on a test or a paper, it only affected you. In many jobs, mistakes can be much more serious. Take your work seriously, and when you do make a mistake, make sure you handle it correctly.
4. Dressing overly casually. I've seen flip-flops, ultralow-rise jeans, visible bra straps, visible underwear, and worse. If you look like you're still dressing for a class rather than a job, it signals that you don't take your job seriously.
5. Using social networking sites or IM-ing with friends throughout the workday. I don't care if you think it doesn't distract you or impact your work; I've watched enough people to know that it does. Similarly, don't text or check your cell messages while you're in a meeting.
6. Dating a coworker early on. Look, I don't care if you want to date someone at work as long as you don't let it affect the workplace—but be smart about it and don't hook up with someone two weeks into the job. It's one thing if you get to know someone on the job over time, discover you like them, and start dating. But jumping into a romance with a coworker you barely know shows bad judgment and makes it seem as if you're more interested in using the workplace as a hook-up source than a place to perform professionally. And yes, people will find out about it.
7. Not understanding the connection between drudgery and more interesting tasks. If you do a great job on the boring work like photocopying and setting up meetings, you'll show that you pay attention to detail, follow instructions, and care about quality. Keep up a sustained track record of that and be cheerful about it, and eventually someone is likely to let you do something more interesting. But do a bad job on the basic stuff, and no one is going to trust you with anything more advanced. So go into it feeling that nothing is beneath you.
Alison Green is chief of staff for a medium-sized nonprofit where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. She is working with the ManagementCenterto coauthor a book on nonprofit management. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.