Ask the police department you are applying with for an applicant study guide or POST sample test or go online to U.S. Capitol Police's website. Read through the study guide carefully, complete the POST sample test and find your score; this will help give you an idea of what to expect on test day and of what test sections you need to practice more on.
Get plenty of rest before the exam day and try not to get too nervous. The POST exam is an aptitude test (meaning it's testing to see what you already know), but test anxiety may make your results not reflect what you truly know.
The first part of the POST exam is simple arithmetic (addition, subtraction, multiplication and division) and requires no law knowledge. Even if the question seems easy, work out the problem on a piece of scratch paper and double check your work so you don't make a careless mistake.
The second part of the POST exam is reading comprehension. To pass, make sure you read each scenario slowly and carefully and make sure you really understand the content before answering questions about it.
The third section of the POST exam tests for grammar knowledge. You will need to be able to identify improper grammar and spelling, so beforehand, be sure you refresh your memory on grammar rules that you may have forgotten.
The last test part, and most difficult for some, is the report writing section based on a fictional incident report. Carefully read the report and answer the questions in complete sentences or they will be counted wrong.