There are a few things you need to do right to ace the Duolingo test. Since it is a relatively new exam with an unconventional pattern, you would need to prepare for it in a slightly different manner.
Here is how you should prepare for the Duolingo test:
Work on your overall English proficiency. Reading high-quality blogs and articles, such as those published in the Guardian, the New York Times, and Wall Street Journal will help you in developing your English skills.
Listen to English podcasts on YouTube or iTunes and other online streaming apps. Make sure you listen to podcasts on different topics.
Try conversing with native speakers. You can easily find partners online - on Facebook groups, for example.
Take the free practice test on the Duolingo website. This will familiarize you with the actual exam pattern and environment. You need to log in to your account to access the practice questions.