I start with a note, just a random note... that usually gets the beast rolling. (The beast drives the car.
)
I too usually have things pop in my head as i am waiting for sleep. I usually have my phone near by and i just record me whistling what ever I hear in my head. the problem is that i listen to it later and it makes very little sense... <_<.
What I hate is that I will hear something at some candid moment, when recording it or writing it down isn't possible, and all i can do is savor the possibilities it could bring, then watch it disappear.