It was during my PMP studying that I stumbled upon a great technique to help me retain the knowledge. I created flash cards - the manual way. I bought a pack of index cards and put the key terms and concepts on the cards. I would then run through the cards whenever I had some free time. I would score the cards by how easily I could answer the question. I would gradually retire the easier questions and work with the cards I didn't know so well. At certain points, I would practice the easy card just to make sure I didn't forget the terms and concepts.
This is known as retrieval practice which is simply quizzing yourself at regular intervals. The best way to do this is through flashcards. The advanced technique is to use spaced intervals to organize the flashcards. This is called the Leitner technique and is easy to implement.
When I started on my second series of certifications, I still used flashcards. The difference is that I used online study card apps. My favorite is Quizlet because it can create flashcards but can also test you through games, quizzes, and easily share your flashcards with others.
The coolest new feature is Quizlet Learn which uses adaptive learning to supercharge the spaced retrieval. However, no matter how high-tech the flashcard, the underlying learning concept of retrieval practice is still a great way to earn those certifications - and to make sure you don't lose that knowledge after the certification exam.