2 suggestions
Here are 2 ideas...
1. This is not a new idea but it works. Draw a house on the board (the usual square with a triangle on top). In the 2 bottom corners of the triangle (roof) write the 2 addends and in the top corner write the sum. You can draw a "road" with many "houses" and make a "fact family neighborhood". Explain that in this neighborhood, each family can only have 3 members. My students have always "gotten it" this way. OOO...you could actually cut out squares and triangles and have the students "build" this neighborhood and make a bulletin board or display of some sort for your observation. Hmmm...I like that idea.
I, too, have an observation coming up...
2. I have those inflatable math cubes with addition (or subtraction) facts on them. We play "Because". We all sit in a circle and I begin by throwing the cube to someone. When they catch it, whichever problem their right thumb lands on is the problem they have to answer AND give the subtraction fact family problem for. For example, if their thumb landed on 9+3, they would say, "9+3=12 because 12-3=9." They catch on very quickly and understand that "because" simply means they have the same family of numbers "working together". (I hope that wasn't confusing.)