I promised myself a day completely off this week, so I'm going to give you a very short and sweet, but extremely effective and useful prompt today.
One of the best things a writer can do is visualize their writing goals for a day before they sit down to work. Things like "I will write four pages," or "I will finish this chapter," are amazingly effective compared to just sitting and writing with no end in sight. This is one of the reasons that NaNoWriMo can be so compelling. It provides a pre-generated goal. (Even the daily goal is specific, measurable, and usually attainable: 1667 words.) Taking this insight to the next level involves setting yourself a specific, measurable, and attainable goal each day before you sit down to work, or even better, at the end of the session before.
We often sit down to write with no real goals. A session can be four hours of diddling our keys and checking facebook and at the end we call it good because we managed to gut out a couple of paragraphs. But when we sit down with a goal in mind it is much easier to have a productive session (or possibly to know we're not going anywhere until we do).
It's the same idea that makes practice sessions with agendas so much more productive than "jam sessions" for musicians.
Prompt- Before you sit down to do any writing today, decide ahead of time what and how much you are going to write. Give yourself a focused, specific goal. Make it reasonable and attainable. (15 pages is a recipe for disaster.) Write only on what you agreed to write. Don't get up until you've finished. Write it down, tell it to your partner in crime, or at LEAST say it out loud to yourself--make yourself accountable.
Bonus- Do this for a week, but decide what you are going to write at the END of the prior day's writing session. As in, "Tomorrow I will finish the chapter I started today," or "I will write four pages." Use the day's writing to gauge a good goal for the next day. You may find that this method works very well for increasing productivity.