I preach all the time to my staff that they need to take control of their calendar or it will take control of them. I constantly struggle with this balance...and I don't think I'm alone. Here's the problem: we believe that if there is an open day, hour, or minute in our schedule, we can (or even should) fill it.
Some fill their schedule because they feel guilty for saying "no". Someone asks us to do something (on a day that is supposed to be a day off, or the last open evening of the week) that seems like a "serving" thing to do; and we instinctively say "yes".
Some fill their schedule because they feel like everyone needs them. We are actually flattered to be asked and that ego builder makes us say "yes" to yet another thing on the schedule.
Some fill their schedule because they don't like sitting still. We are addicted to activity, and so the more we can pile into our social and activity life is an absolute no brainer.
The problem is that we are exhausted, late, burned out, tired, grouchy, ineffective, and frankly unspiritual. Why you and I should say "no" to events and stuff on our calendar than "yes".
Saying "no" to the stuff other people think we should do allows us to say "yes" to the stuff we have discovered God made us to do.
Saying "no" to helping everyone and being a mini-savior allows us to be content to say "yes" to stuff that only God can do through us. (ie. there is only one savior and he wants to use us for his kingdom, but he doesn't need us to do everything).
Saying "no" to constant activity allows us to slow down and say "yes" to spending time with God.