I've recently taken on more than I can possibly handle (this is a case of do-as-I-say-not-as-I-do). It happened for a simple reason: there are lots of tasks that take time, but that don't make it into my planning. My schedule appeared to have a lot of free time, but it turns out that it had far less ... because many of my uses of time aren't the sort that get put on a calendar.
Before scheduling something new, take a moment to mentally imagine everything that will happen during your day. Pay close attention to the things that you don't put on your calendar. Some people forget travel time, others don't write down meals, and others don't plan for sleep. Make sure you either put them on your calendar or leave enough blank space so they actually get handled. I now try to put them all on my calendar, so I keep time not only for the visible tasks, but also all the invisible ones.
For tips about the invisible tasks and how they affect being on time, check out my article about how to be on time.
I'm Stever Robbins, host of the Get-it-Done Guy podcast. I help executives reclaim their time by providing focus and accountability around their most important goals. If you want to have one of your most productive days ever, try one of my Do-It Days™: http://www.DoitDays.com.