I remind my kids to think if the apostrophe works as a band aid or a lasso. If it doesn't, it's not needed.
I don't claim the band aid story. My students gave it to me, but it sure is clever! They told me the apostrophe is like a band aid in contractions. Since the two words were squashed into one, some of the letters popped out, and the band aid is needed to heal the spot where the letters popped out.
I do claim the lasso story. When teaching possessives, I make sure the kids know the word "possess" means to own or have something. I'll get into stories of rodeos, telling them how cowboys throw their lasso and claim their cattle. I show them pictures I've googled of cowboys and lassos. In a possessive, the noun with the 's owns the following item. I even get into turning the apostrophe into a lasso and circling the next word. They practice this on their whiteboards (I'm a whiteboard fanatic!) and love to draw the lassos.
Naturally, if the word they're thinking about doesn't need a band aid or a lasso, they shouldn't be using an apostrophe.
We know how these little stories help the children remember. After 34 years of teaching, I have lots of little stories and "tricks up my sleeve". Recent brain research shows us these little stories help make the connections in the brain so the children can build their knowledge. Plus, it's fun!