Sometimes, saying “No” definitely is in order. Examples of these times include the following:

(a) when you cannot do the task (lack of time, inadequate qualifications, inability to be present at the time needed), (b) when the request is a low-priority item such as a C project, (c) when it is a waste of time (bad ideas, unimportant issues), (d) when someone else can do it better or when it is someone else’s job, (e) when unrealistic deadlines are imposed, (f) when the request is illegal or unethical, and (g) when the request compromises your own integrity, even if it is not illegal or unethical.  At times, asking if the requesting party wants you to set aside everything else to work on this project may be appropriate. In this case, specifically mention the other projects that you will have to set aside.