Many prospective employers want to see a legal writing sample and will use the sample to evaluate your ability to effectively research and analyze a legal issue. The overriding consideration, when it comes to a writing sample, is that it be your very best work and reveal your ability to clearly analyze case law or a statutory scheme. Spend the time to make your writing sample impressive. Many employers consider it a very important indication of your potential as a lawyer.
Your writing sample should be in finished form (no typos, corrections, or professor's comments). Ideally, it should be six to ten pages in length. Longer samples can be excerpted and submitted with a preliminary summary page that outlines the facts, making it simple for the reader to immediately grasp the subject matter.
Legal briefs, legal memos, moot court briefs or work product from a law school course (e.g., legal research and writing brief/memo or seminar paper) are all appropriate writing samples. Employers often like to see something you wrote on the job, since it is usually written under real-life time constraints and, therefore, more representative of the kind of writing you would submit if you worked for them. If you wish to use a legal brief or memo you wrote at work, however, you must get your employer's permission and delete any confidential material. You may insert "firm name" or "client name" to protect confidentiality while maintaining readability.
Finally, avoid submitting a writing sample that has been heavily edited by a third party. For example, prospective employers will generally not want to read your law review article, as they know it has been edited extensively. If you do decide to submit a law review article, you should also submit another, self-edited piece.