How to Say No to the Demands of Others

This article from Mediate.com excerpting The Power of a Positive No by William Ury,

offers specific key words or phrases you can use in saying No to the other's demand in a way that flows naturally from your Yes, your power, and your respect. Remember that your tone and underlying intent need to be congruent with your words if they are to have the right impact...

"No" or "No Thanks"...
"I Have a Policy"...
"I Have Plans" or "I Have Another Commitment"...
"Not Now"...
"I Prefer to Decline Rather Than Do a Poor Job"...

In short, know your limits, acknowledge them freely, and spend your time on what you can do well. Both you and the other will be better off in the long run.