Mind mapping is tremendously useful technique for getting thoughts out of your head.
If you struggle with gaining clarity on big project that you need to run, trying to make a big decision, writing a book mind map will help you out. Mind mapping lets you capture all the ideas about a topic and then find a structure based on the common themes, connections etc.
Lifehack.org share extensive guide on how to use this tool which is worth checking out if you are not familiar with it.