I have spent years listening to people telling other people to “lay down”. I don’t often correct them. So for all of you who say that regularly, you really mean to say, “lie down.”
Lay is past tense. Lie is what you are telling someone to do in present tense.
When I was in school, my teacher told us to remember that chickens lay, people don’t. That was memorable for me. Now when I hear the lay/lie confusion, I picture a chicken laying an egg. I can’t stop the image. It just appears in my head as if urging me to correct the speaker.
Here’s another way to look at it. You can lay something down (as in “chickens lay”), or you can lie down by yourself.
I hope this helps clear up the confusion. Happy proofreading!