To keep track of tasks, I have a little document called a task list. And in the same document there's a list for each person I work with or interact with, of what they're working on or what I expect from them. It's just a list in a text file. Using this, I can plan my day out the night before: "These are the five high-priority things to focus on."
Mayer receives 600-700 daily emails and sits in 10-11 hours of meetings a day, and uses a simple text file to manage her to-do list.
I have a similarly ultra-simple approach: stickies on the Mac. It is equivalent to a text file. Different stickies hold tasks of varying timelines and priority. For example, "Long Term To-Dos", "General Short-Term," and the most important, daily sticky created each night and morning, "Thursday Tasks." When I finish a to-do, I delete it. I love stickies for how easy they are to manipulate and how fast the 860 kb application runs off my desktop.
Too much complexity is the problem with sophisticated task management applications. I don't want to have to fill out (or look at and choose not to fill out) various fields. Not every task needs to be dated. I don't want to categorize my tasks, or if I do I want to do so on the fly using basic formatting like bold or italics. Over-optimization is a common trap in the organization and productivity and lifehacking world.
I supplement my use of Stickies with the "Tasks" and "Calendar" functions of Exchange Server (which I access via Entourage). If I have a time-sensitive task that I do not want to think about until I need to do it, I will create a Task and attach it to a date. For example, if I'm meeting with a guy next week and want to remember to bring him a book that's on my bookshelf, I will set a task to remind me two hours before I leave for the meeting to grab the book. If I have a super time-sensitive task that I do want to think about in the time before it's due, I will add it to the appropriate sticky and add it as an event on my calendar on the day.
Finally, I have "temporary to-do lists" on my mobile device and in the notebook I carry to all meals and meetings. Per David Allen, anytime a task crosses my mind and I'm away from my desk, I jot them in my notebook or on my mobile, and then once a day transfer the tasks into my main Stickies set-up or into my Exchange calendar.
Bottom Line: Find a system that works for you, and everybody is different, but beware of overly complex task management systems. Even really busy people like Marissa Mayer do just fine with a text file.