Before You Commit
Once you've made this mistake once, you never want to make it again. It's the mistake of agreeing to be in two different places at the exact same time. In college I booked our brass quintet for two different gigs on the same Sunday morning. Having to break one of them was mortifying to say the least.
I learned quickly how to be extremely detailed when it came to scheduling and calendars. I never wanted to repeat that error, and I never have. And it's one we should all strive to prioritize. Breaking a commitment, especially one that has been settled for some time, will also break trust and confidence in your ability.
Put everything in your calendar: work events, family time, obligations, and even tentative meetings. When anyone asks of your time or presence, never agree without consulting your calendar. Be sure there are no conflicts or chance that you may be unable to fulfill this new commitment. Then once it's agreed upon, place it on your calendar. Immediately.