1. Are your tasks clearly defined? Lack of clarity can drop the enthusiasm level.

2. Are you excited about the tasks? See above.

3. Are you clear on the reward for completing the tasks?

4. Is there simply a rhythm your body and mind WANT to work in, that you'd likely be best to listen to? I'm a "Grind It Out" guy and work on things to death once I start, but some people really like Pomodoro, for instance.

Also, we all have our "Golden Hours," a topic I was quoted in Inc. Magazine about...times during the day when we're best suited to do certain work. In my case, I am best at "grunt" work in the morning...non-creative, bang-it-out type stuff. And I get tired from about 3:30PM - 6:30PM so I try not to book anything but podcast appearances in there (which instantly put me on Performer Mode.) Then from about 7:00PM to 1:00AM I am creative, so I schedule those tasks then. Respecting these preferences is important to my success. Can I do creative copywriting in the middle of the day? Sure. But the end result will probably not be as inspired as it could be.

As Dan Ariely said, we don't know our preferences that well. If you learn yourself, you can use that machine to its best advantage.