You know the best way to guard your time is to say no. Trouble is, you don't always realize you should've refused a project until after you've agreed to do it. How to ease out of a commitment, whether it's a job assignment or a school bake sale:
Negotiate small changes with big payoffs. Meetings eat up time, for example, so see if you and a fellow team member can switch off to attend them, or request a temporary reprieve from other meetings.
Bring in reinforcements. You might delegate a task to a junior staffer who wants to be more involved. Or if you can no longer, say, chaperone a field trip, barter with a friend: She goes on the outing, and you'll watch her kids on the weekend.
Get perspective. It's better to drop an obligation -- especially if you can't give it the attention it requires -- than to let it turn life into a pressure cooker.
Minimize the damage. To break the bad news, you might say, "I thought I could pull this off, but I realize I've spread myself too thin. Since I'm not performing up to my usual standards, I think it would be best for me to step down." Avoid the temptation to swear you'll take on another task later to make up for it. Chances are, life won't be any calmer then.