If you stretch out on your bed or your couch, you LIE down.
If you put something on your bed or your couch you LAY it down.
There: is that so hard to keep straight?
Lately, I've heard even respected journalists make the mistake of saying "lay" when they mean "lie."
So, at least on this blog, I am laying down this grammatical law for all to see.
And now, I shall go into the adjacent room and lie upon my couch.
I could also lay my self down upon my couch, but in this instance,
I prefer to be true to lying.