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.
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.