Avoid prompts that are complicated, highbrow or require a keen knowledge of current events to answer. This will stymie students' creativity and diminish the quality of their writing sample.
Strive to write open-ended, thought-provoking prompts. Those that inspire little more than "yes," "no" and "maybe" answers will elicit limited information.
Take a careful measure of your students and then write your prompts accordingly. Consider their demographics (gender, age, marital status) and psychographics (interests, hobbies, personality and lifestyle). Your writing prompts should be crafted much differently for a class of 18-year-old college freshmen than a class of adults in their mid-40s. Your prompts should be interesting and tailored to your audience -- make it something they can relate to.
Write prompts either in a question format or as the topic sentence of a paragraph that students should elaborate on in at least four complete sentences. A prompt for a class of 18-year-old freshmen might read: "If I were 40 years old, I would raise my teenager much differently than my parents have raised me." A prompt for a class of 40-year-olds might read: "If I were 18 years old again, I know exactly what I would be doing this weekend." A question-style prompt for either group might read: "If money were no object -- and you didn't have to worry about making money to support yourself -- how would you spend your time every day?"
Write the prompts on a chalkboard or whiteboard or distribute them so students can read them carefully before producing their writing sample. Most students are visual learners, so they are apt to do better with the writing sample after reading (instead of merely hearing) the prompts.
Give students a choice of which prompt to respond to. Encourage students to integrate dialogue into their prompt, if they choose; this will give you another good insight into their ability to form and write ideas. If you give them a choice, say, of three good prompts, there should be at least one that inspires that "I can write about this" reaction. Then allow them between 20 and 30 minutes to do just that.