"7 Idea Dumping Tips (How To Manage Diarrhea of the Brain)" has some great ideas on capturing ideas -- always carry paper, and so on.
I use brain dumps** when:
1. I feel restless. I've learned that this means that I've got an idea (for a project I'm not working on, or for a new project), and the idea is trying to hatch;
2. At the start of a project. Whenever I get a new commission, whether it's from a copywriting client or an editor, I do a brain dump. I give myself 20 minutes to just write anything and everything. This is always useful immediately because it gives me confidence, and often later in the project -- I can go back and mine the brain dump for ideas;
3. At the end of a project -- this is always when I get ideas for new projects, based on a nugget in the project I've just completed;
4. When I'm frustrated with a project that's in a rut.
** Brain dumps = literally writing what's on your mind -- even though you don't know what's on your mind until you write it. Give yourself around 20 minutes just to write on a topic. A brain dump isn't free writing, where you just keep writing: with a brain dump you stay on the topic.