If the above ecard from someecards resounds with you, chances are you've experienced Parkinson's Law at work. Parkinson's Law states, "Work expands to fill the time available for its completion." Essentially, the time you allot to doing a given task is exactly how long it will take you to do that task.
I fight this by seting myself challenging deadlines and setting attractive rewards for successfully completing them. For example, if I finish my whole day's work before lunch, I get the rest of the day off to spend as I please.