Here's a tip from Discardian Joe:
In high school, I was good friends with the tech director in the school's theatre. He had a lot of commitments, in his job at the school, in local theater, and in the community. But he had a remarkably good method of deciding whether to take on a design job or more volunteer work. He decided how much his free time was worth to him, in dollars per hour. If the new commitment wasn't either remunerative enough or fulfilling enough to be worth that amount per hour, he said no. "They were going to pay me $500 to do the set design," he'd say, "but it was going to take 25 or 30 hours of work, and I bill my free time at $25 an hour."
Decide what your free time is worth to you. If you're an undergraduate, the number might be around $5; if you're a CEO, it might be in the hundreds. Before you take on an optional commitment, ask yourself whether the compensation or pleasure of the task is worth the amount of money it'll cost you in free time.