Jason Fried, Be More Productive. Shorten the Workweek:

When there’s less time to work, you waste less time. When you have a compressed workweek, you tend to focus on what’s important. Constraining time encourages quality time.

I’ve found that to be true not just on the week level but also on the day-to-day level. Limiting the number of hours you’re around in a morning, afternoon, or entire day forces you to really focus.