> It might be extremely personal, but my best trigger was imagining walking down an infinite stair above a cloudy sky right before sleeping - imagination transitioning into dreaming as it went.

I remember doing almost this exact thing as a child! Every night I'd fall asleep thinking of descending an infinite escalator, which existed in an empty space. Sometimes I was with other people, sometimes alone, but I found that by keeping the escalator in focus, I was able to switch into what now I realize was a lucid dream, right as I was falling asleep.