Decide what information you deem imperative that students learn throughout the course of a unit of study. In many cases, you teach an enormous amount of information in one unit, but not all of it is necessary for students to retain. For example, you may teach a variety of strategies to help students multiply, but they only need to understand the process of multiplication, and they can pick and choose the strategy they use.
Plan objectives that clearly state what it is students should be able to do with the information they learned upon the completion of a unit. For example, a learning objective for math may be, "Student will be able to multiply two-digit and three-digit numbers."
Write test questions that clearly assess a student's ability in regards to a specific objective. For example, if you expect a student to be able to identify three major causes of the Civil War, the post-assessment should have a question that asks students to specifically do that.