I just learned an easy way to remember Physic weaknesses, they're the three fears. Bugs, darkness, and death (ghost)! I could never remember bug, this is really helpful to me.
I guess that's one way of remembering it. However, people have different fears such as heights (flying), thunder (electric) and war trauma (fire). To me, I've played pokemon ever since I was young so I have most of the weaknesses remembered.