It is a beautiful thing when your time is so organized that you know what you can do and when you should say no. It is important for those of us who are busy to recognize when we may be over committing our time. If you do not know where your time is spent and you take on too much, then you will not be giving your all where it is needed; it is a disservice to your clients, students, children, customers, family, etc. when you over extend yourself. When your schedule is clearly defined and you have your priorities, set then it is easier to say no.
Along these lines it is important to recognize that it is okay to say no. You do not have to do everything that is asked of you. Clearly set your priorities. Block out where you want/need your time to be spent and stick to it. It can actually be more rewarding when you say no because you know you are being true to your priorities.