When I introduce this tricky concept to my students I give them this visual organizer to try and help them separate the two forms of "to be." I use P.I.E. for Ser and Pepe Loco for Estar.
I illustrate Ser with an outline of a man and a question mark and a clock.
I illustrate Estar with a picture of the water cycle. I explain that H20 is still H20, no matter if it is ice on the ground, water falling down, or vapor in the sky. Likewise Estar is a state of being or the location of something.