I'll often get hit with solutions to ideas at the worst times to write them down:
In college one of my instructors said that this is the reason why it's always best to start projects as soon as you can, because a creative mind is never truly "off".
If you have 3 projects, don't do them in this sequence:
project 1 -> project 2 -> project 3
But rather, even if you only spend a few minutes do this:
start project 1 -> start project 2 -> start project 3 -> finish project 1 -> finish project 2 -> finish project 3
Because after you've started it, it sits on the back burner, you might not even be aware or conscious that you're thinking about it, but maybe you'll be eating dinner and an idea will hit you. But that won't happen if you haven't even started the project yet.