Create your "absolute yes" and "absolute no" lists. Cheryl Richardson, author of the 2009 book "The Art of Extreme Self-Care," makes the case for such lists to determine what is and is not important to you. To create your "yes" list, set your top priorities (family, community service, work projects, emotional and physical health). When people ask you to do things related to those priorities, your response can be a "yes." If, however, the request isn't related to your list, you can say "no." Likewise, create your "Absolute No" list (things you no longer do, no longer want to do or would like to give up in the future). For example, maybe you put on your list "I no longer check emails after 9 p.m. or on Sundays," or "I no longer accept phone calls during family dinners." Having such a list and looking at it daily can remind you of things that are definite "nos."
Set "no" as your default answer. Instead of starting with "Yes, I will see if I can do it," start with "No."
Pause first. Put some space between the request and your answer. I've worked with a colleague who is really good at this. When people ask him to serve on projects, he firsts asks very good questions about the nature and scope of the work involved. Then, he says he needs time to think about it given his other commitments. After he has spent some time learning how much work is involved, he then offers his response.
Let the person know from the beginning that you may not be able to help them. Point out that you have a number of other commitments and will need to review these first. This also lets them know that it might be a good idea for them to look at other options rather than simply relying on you to do the task.