Always pack for the occasion. If you suspect the employer may want a writing sample -- or the interviewer asked you to bring one -- choose a sample that is relevant to the job for which you are interviewing. Always ask the employer to be specific about what you should bring. Should they leave it to your judgment, bring your best work.
Bring Something Tailored
When a tailored sample makes sense, create a writing sample that demonstrates your relevant skills and qualities -- specifically. Ask how the writing sample will be evaluated; it might be assessed on content, expertise on subject matter, style, grammar, communication skill or personality traits. Spend more time developing your content or topic if it is the focus. Otherwise, focus your efforts primarily on the writing mechanics and worry less about the topic. Always choose a topic that is appropriate for the workplace and is not controversial or propaganda.
Bring the Right Document Type
Think of reasons why the employer might want a writing sample and what they will learn from it. If you are applying for a writing job, it's purpose is straightforward -- the employer wants to know you can write. For other jobs where the connection is less clear, you may have to think outside the box. Perhaps the employer wants to know that his assistant can write in a professional tone or he wants to know that his sales representative is a persuasive communicator. If the job involves creative writing -- bring a creative piece. If the job requires technical writing -- bring a technical piece. If the job requires you to write annual reports, grants, business letters or newsletters -- bring a sample that demonstrates your talent in those areas. Law jobs require someone who can write about law.
Don't Reinvent the Wheel
Your perfect writing sample might already be in your collection or on your hard drive. Perhaps your thesis or another academic paper will fit the bill for a research job or teaching job. Select a few pages of a longer document -- a sample should only be two to five pages long, unless it is for a law interview. Have permission to use something from a previous job and omit any confidential information. Consider a writing sample you created for your last job search. Existing content must be relevant and should not date you.
Bring Your Best Work
Always bring your best work. Bring a writing sample that impresses you, or has impressed a past employer. If you have trouble choosing a sample, ask for feedback from a colleague or friend. The sample you decide on should be free of errors and demonstrate the highest quality writing you are capable of. This rule can serve as a tie-breaker; if you cannot decide based on content or topic, base it on writing quality.