We had a great conversation at our offsite last Tuesday on the importance of being able to take on a reasonable workload, avoid overwork and burnout, and to know when to say “no”. There are both more and less effective ways of saying “no”, especially to your manager. It is generally not a good idea to “Just say no,” but instead to let your manager know your priorities and what the sacrifice would be if you did this new project instead.
Here is a resource from Forbes (in pictures!) on how to say “no” at work that explains this technique and others.