I let my head wander through a land of dicks, pussies, panting, orgasms, and thoroughly naughty things. I snuggle into my husband and let my mind play through dirty scenarios, imagining things that turn me on, and wondering at things I'm curious about. I meet metaphysical characters in my head in the middle of the night, playing around with qualities, with quirks, flaws, and kinks.
In some way each of my characters is linked to me in those moments and it's from those characters that the stories are built, the ideas grow and mature.
Brainstorming is one of those things a lot of writers realize they should do but don't formally do. Sitting down and letting your head wander with no distractions. There are a lot of excellent brainstorming activities out there - anything from free writes, to setting the scene, to starting in the middle. But sometimes you don't need the structure or sitting down and saying "Now I am brain storming."
Letting yourself weave through ideas in a freestyle form can be more constructive than sitting down and attempting to force words out on the page.
So the next time you have insomnia, are stuck on a long car ride, or are just plain bored, let your mind wander through sexy thoughts, and maybe you'll stumble on a sexy story.