Ser and estar both mean "to be". In order to differentiate the use you must first decide if you are speaking about a characteristic or a condition.
e.g. La manzana esta verde - The apple is unripe. You are speaking about the condtion of the apple.
e.g. La manzana es verde - The apple is green. You are speaking about the characteristic of the apple.
I find that placing it into a chart helps the middle school/high school aged students to better format it. Also no memorizing is really going to help, but more practise than anything else.
Actually, my teacher taught us an even more specific acronym -- it is still Dr. Elf, but instead of Dr., we use Doctor.
I hope this helps!
use DR ELF description, relationship, emotions, location, feelings
The easiest way to get to the initial difference and to keep it simple is just to tell SS that SER es PERMANENTE and ESTAR es TEMPORAL.
Ser is to describe personality (PERMANENT)
Estar is to describe moods (TEMPORAL)
I break it down by how you are and where you are vs. who and what you are.