Saying "no" is a very reasonable response to any request no matter how big or small. If you can't do what is being asked or even just don't want to, it is truly okay to say "no". No further explanation is required. The fact that you don't want to is more than enough reason. If you are a person that has trouble saying "no" outright then respond by saying "I'll consider it and get back to you". This is alright too. It is much better to give no commitment at all than to commit to something you didn't want to do in the first place. Mastering the art of delegation is another way to lessen the burden of being so agreeable. Don't be afraid to request help from others of similar competence. By delegating tasks that you are either unwilling or unable to perform, you can focus your attention on matters that are more pressing or important to you.
There is so much additional stress that develops when a person attempts to perform a task unwillingly. Think about times when you have worked at something you truly enjoyed. How easy was it for you to complete the task successfully? Now think about a time when you have agreed to a task that was not really your cup of tea but you just couldn't say "no". Did you give the task your complete 100%? Was the task completed and completed on schedule? How did you feel during the process?
Saying "no" firmly and asserting your position can give you power beyond your wildest dreams. Speaking "no" is to overcome your fear of rejection. It is to embrace independence. It is to get in touch with your true values. What do you stand for? How do you want to live your day? Saying "no" gives you back the control over how and with whom your time and energy is spent. So where does one begin? By just choosing the positive power of saying "NO".