Listen to fully understand what is being asked of you.
If your decision is no, say so politely but firmly.
If appropriate, explain your reasons.
Suggest other alternatives to resolving the other person's
problem or
need.
Some possible responses:
"I have other plans"
"My schedule just won't permit it"
"I'm doing something else that night"
"I have too many commitments right now"
Some alternatives for ending calls:
"Thanks for calling"
"Thanks for thinking of me"
"Thank you for the invitation"
"I appreciate your calling me"
"It was nice talking to you"
--"The Time Trap" by Alec Mackenzie