In Secrets of Productive People I recommend that you have a list of authorized projects (rather like a firm has a list of authorized tradesmen).
There are basically two reasons for the list, one a positive one, and one a negative one
The positive is to keep you aware of all the calls on your time so that you can ensure that each project receives adequate attention.
The negative is that a project is as much about what you are not going to do as about what you are going to do. The list therefore keeps you focused on the projects on the list and stops you from deviating onto other “bright ideas”. You should not allow yourself to put any task on your to-do list which doesn’t relate to one of these projects.
The Authorised Projects list also helps you to avoid over-committing yourself. If you want to introduce a new project you need to be able to demonstrate what projects have ceased or have been weeded out in order to make room for the new project.