Late last week I read this article "To-do lists don't work" published on the Harvard Business Review by Daniel Markowitz. Markowitz's thesis is that todo lists don't work because they leave too much of the prioritization of tasks to the "last minute mind".
In other words you have a moment, you look at your list, and you make the wrong choice about what to work on because it's the shortest todo, not necessarily the most urgent. Markowitz recommendation is not to work from a todo list, which you still need to queue up tasks, but to translate those tasks to a calendar and work from that.
This technique allows you to allocate time to do tasks on a calendar and not overcommit yourself. It also makes it plain when you do not have time to promise something based upon a quick glance at your schedule. It forces you to say "If I do that, then what has to get moved off". My expectations of what I can accomplish in a week have been tamped down to reality, and my confidence that I can get to the things I plan has skyrocketed.
Additionally, I don't end up working too long on a task and have it steal time from another project without thinking very clearly about the impact of that.
I realized pretty quickly that I could pull this off using Google Calendar. Though I work at home, I have a pretty busy interlocking schedule of conference calls, dedicated periods where I pull and analyze data weekly, and various child duties (school drop off, pickup, OT, etc).
I turned on a few overlapping calendars in Google (Kids schedule, work schedule, and US Holidays) and then set my "working hours" so that only times from 7am to 7pm would display. I printed out a 5 days calendar big enough for writing in the 30 minute slots, and set to work. Each day I'd plan what I was doing by filling in the calendar. Sometimes I would plan something for a few days from now.
The system worked great. While I still have a todo list to remind me of tasks to do for different clients, I feel far less guilt about what I'm getting done because what is scheduled gets done. And at the end of the day, I know why.
I recommend you try this for a week. Its a painless exercise and will likely make you more effective.