I am writing a book on this topic.
1. Have a storage bin for each project. (I use stackable plastic bins of the style you might find in elementary school classrooms. I label them by making masking-tape flags that hang off the side, and then writing on the flags with permanent marker.)
2. Keep your drafts in the bins, but also put any loose notes/sentences/paragraphs/etc. that are related to the project in the bin. Thus, the bin serves not only as the project's physical storage, but also as its inbox.
3. Work out of a notebook with pages that you can tear out; or carry around notecards or other scrap paper; or grab whatever paper is at hand when you need to write during the day. But the key is to write each discrete idea (or sentence/paragraph/etc.) on a separate piece of paper. Go about your day, writing whatever you need, but keep everything you write together.
4. At the end of your day, distribute the jots/notes/drafts into the corresponding bin.
5. When it's time to work on a particular project, grab the corresponding bin. Your drafts will be there, but so will the loose materials you have created. Incorporate the loose notes into new, combined jots/notes/drafts.
Repeat the process until the project is complete.
Ideas rarely come to us all at once according to the project to which they belong, or in the proper order. The beauty of writing discrete notes on loose papers is that it's easy to capture them and then later put them in the correct bin; and when working out of the bins, it's easy to move, group, stack, and otherwise organize or delete the ideas — working with them until your project essentially writes itself.
Happy to share more of my own experiences and suggestions if you have questions. Regardless, please let me know if you find this helpful or have any feedback.
Cheers!
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