My best ideas always come when I'm not paying attention ... or conscious. A large percentage of them happen while I'm asleep. It's as if I watch the entire story play out in my dream and then I wake up and think "I want to write that".
Of course, the whole story never sticks with me (and I suspect it wouldn't be terribly good if it did and I could look at it with the part of my brain which doesn't automatically accept carrots as a viable means of transport on the way to the bird show at the library). But there are usually some scenes which stick with me and those form the nugget for the story I will actually write.
I have to write down details as quickly as I can. I usually keep a notebook on my bedside table for just this purpose. I don't bother with names or try to improve on anything, I just write down what I remember. I found my original notes for Revelations and this is what they look like:
Heroine is a burlesque dancer who can enchant men into seeing their true selves. She has mystical powers as part of a long line of priestesses. She is due to take up her mantle as High Priestess, which would mean retreating into secret temple.
Hero wants her to stay. He needs her help to track down his sister who is lost on city streets.
Something dark and dangerous is in the temple. Something which would devour the Heroine whole if she becomes High Priestess. Like the minotaur (?).
Obviously the story has come a long way since that first dream. But that was the spark which got it all moving.