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25 minutes of concentrated work, followed by a 5 minute break, another 25 minutes of focused work, followed by another 5 minute break. (After 2 hrs, take a longer break before beginning again.) Breaking a larger task into smaller pieces makes it more manageable and the timer helps encourage undivided attention.
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This technique was voted as the best productivity method on Lifehacker.com in 2012. If you want more information, there's a Lifehacker article here and a Pomodoro website here, but the simplicity of the technique is what contributes to its popularity.