Many systems have been devised to organize work into several priority queues, calendars, etc., with sophisticated tracking tools and graphics. Here’s instead the simplest possible effective way to get things done.
Write down every idea, thought, request, wish to do something, all in one place. This avoids getting yourself overwhelmed by too many sets of things to do.
Then, go over the list and split it into two lists, titled “Yes” and “No”, according to the answer to the following question:
Is doing this absolutely indispensable in order to finish this project?
The Yes list will contain tasks crucial to the purpose of what you’re doing, while the No list will be full of “nice to haves” and interesting ideas. You can do this either all at once, or in batches of 10 items, or even for each item separately, when you start working on it.
Until the project is complete and delivered to where is has to go, only work on the things from the Yes list. If anyone asks about their request that you’re put on the No list, tell them you’ll get to it later.
You cannot focus on more than one thing at a time, although you can [try], at a great cost to your sanity and productivity. The Yes list enforces your unity of purpose.
Then, hopefully, the project is out: paper published, product shipping to customers, system running and being useful. Do not discard the No list just yet! It’s full of potentially good ideas which you have subconsciously been working on (in [true parallelism] with the main project) for many weeks, months, even years. Scan it for these ideas, form new projects out of them, present or assign them to others to work on. Of course, many of these will be obsolete by now, or simply not good. Graciously let go of them — throw them away.