What works for me is to just put on a documentary and something about the monotone narration voice just shuts my brain down. I don't even pay attention to the words. I'm generally asleep within minutes. I assume audio books might have the same effect.
I guess it might be related to my young childhood as I had parents who would almost always read to my brother and me at bedtime. So possibly there's a part of my brain that has associated falling asleep to the sound of a voice speaking.