There is no single format for writing samples, as it depends on the university department or company involved. Purdue's Online Writing Lab (OWL) provides directions and samples for a wide range of writing tasks, and WikiHow.com offers guidelines on choosing the right material.
Usually, the two most important components of a great writing sample are insight and proofreading. Your readers want to see that you have something real to say and that you are adding to the conversation.
Choose a sample in which you show that you understand the subject and have your own insight. Then, edit and proofread carefully. Look for errors in grammar, spelling and word usage. Strive for dynamic language with active verbs. Double check for plagiarism or copying. They want to see your writing and your personality.