STATE YOUR PRIORITY
Figure out how much time you actually have and how you want to use it. Susan Thomsen, a software engineer and mother of four in Dayton, OH, spends her five free hours per week in her kids' elementary school, planning its annual auction. She turned down a request to become the vice president of the Dayton Bar Auxiliary (her husband is a lawyer), since it didn't benefit her children.
You can say that you have only a few hours each week and your children's school/church/scout troop is where you need to put them.
CUT TO THE CHASE
If you know right away that you can't take on a job, say so promptly. Don't torture yourself -- or raise the hopes of your caller, who hears "Let me think about it" as a potential "yes."
KEEP IT SIMPLE
The less wordy your explanation, the better. "You needn't say why you can't take on a task," says Martin. "Just keep repeating, 'I'm terribly sorry, but I can't.'"
Complimenting the cause or the caller can help you over the hurdle. Say, for example, "I have such admiration for the work you're doing, but I can't commit my time to it."
SAY NO, BUT....
As a last resort, turn down the Big Job (heading the school fair) but take on a piece of it (running a booth). Or make a donation -- of a raffle prize, cookies -- just not of your valuable time.