When the initial little impulse comes it just tends to come. I wake up in the middle of the night, or I'll be swimming in the pool with my kids and just think, "Can't forget that one. " Then there's the more organized side of the brain, which is when I choose to work on them. I've learned through a bunch of mentors how to demarcate the different times for your writing process. I keep multiple projects going at once and there's a time when no matter what you're doing, you have to stop and write it because it's coming.