People has different ways in memorizing things. In my case, I let myself first understand what I am reading. After that I list down all key terms and start asking myself what those terms are about. My advice is you do not just memorized something, you also have to fully understand it. And with that i assure you it will took years before you forget it.
Kingdom, Phylum, Class, Order, Family, Genus, Species.
Sounds familiar? I still remember many things I learn from high school even my ex i still remember everyday. hehe. *Except grammar*
Focus and stay on track. Goodluck.