Thank you, Luis, for starting a discussion on this important topic! I still struggle with saying "No", but one of your bullet points resonated with me - "Is this something only I can do - or could it be delegated to someone else?" We now have two graduate engineers and if I am overloaded with work, I will suggest delegating the work to them. The key point I make in these situations is that I state that I am happy to make myself available if they have any questions about the work.
We also have a weekly bridge group meeting every Monday where we review a spreadsheet that included everyone's anticipated hours for the week. Of course, things pop up throughout the week, but generally this Monday meeting does help us spread the work evenly throughout the team as well as set work priorities.