Watch a shit ton of surrealist bullshit. You are bond to dream whatever is floating in your mind at the moment.
Try daydreaming while still in bed awake.
I don't think this is necessarily true. I often tend to dream about things I haven't thought about in years. It's rare that the imagery in my dream will follow a linear logic of what I saw or thought about during that day.
What's the source of that image?
Also, since diphenhydramine takes about 2 hours for its maximum effect, do you take it right before sleeping or just some time before?