First of all, be sure to look around and identify any hot topics within your own department. If the administration is concerned about the frequency of a specific crime or with a specific problem within your city, you'd better brush up on it because you know it will appear on the sergeant's test. Study the problem and come up with some basic ideas of how you would combat the problem or issue before walking into your test. That way, when the question pops up, you'll look prepared and ready to be a supervisor, not like an officer fumbling for an answer.
Second, be sure to study some basic scenarios you could (and probably will) have to handle as a sergeant. The frequent ones that appear on tests are counseling an underperforming employee, a large-scale crime scene or multiple crimes scenes, and an irate or concerned citizen. How would you handle these? How do good sergeants in your department handle them? Study up on these and other common supervisor issues, and be ready when they appear.
Talk to High-Scoring Sergeants in Your Department
This one is the most frequently overlooked activity of all sergeant test study aids. Be sure you speak with the sergeants in your own department who recently tested and scored high on the test. How did they study? What did they study? How did they score so high on the test when others didn't? These sergeants have unique insight that can help you in your own testing process. Don't look at this as "kissing up," it's actually far from it. If you think you can be a lone ranger throughout your career and not ever ask for help, then maybe you'd better consider whether you'd actually make a good sergeant. We all need help occasionally, and this is the perfect opportunity to seek it from someone who has a very unique ability to give you a boost.
Along the same lines, speak to lieutenants and above at your department about any test taking tips they may have as well. Remember, these are usually the same people actually making the test. Although they can't give you the answers, they can definitely steer you in the right direction to maximize your study time!