Motivation is a requirement to keep you focused on achieving anything you'd like to do. Sometimes motivation slacks and you find yourself procrastinating. You have a bad day so you tell yourself you'll get back on track tomorrow. Tomorrow keeps moving farther out and soon you lose all motivation. Then doubt and depression settle in. This is true for every aspect of life, but especially true for writing.
When I joined Weight Watchers I looked at where I was and where I wanted to be. The numbers were so far apart, that I nearly gave up before I started. However, WW suggests setting a 5% of your current weight as your first goal. So, now I'm looking at an achievable goal. I'm motivated as I draw closer and closer to the first success platform.
The same thing is true of writing. When you look at needing to write 300 to 400 words and you are on page one, the end seems so very far away. Many writers balk at setting goals. Life gets in the way and nothing gets written for a day, or more. Now, the writer feels "behind". Then it becomes easier to miss another day. Doubt and depression settles in. This, my friends, is a death knoll. I know. I've been there.
So, how does a writer establish attainable goals to keep them on track? The easiest way is to establish a goal to write "something" every single day. Even on a "bad" day, you should be able to write one sentence. Establishing a routine of writing every single day leads to producing more pages. Some days you'll do a lot more than others. Some days you may only write one sentence. But, as long as you've written something, then you are still on goal. Each word leads to a sentence. Each sentence to a paragraph. Each paragraph to a page.
Allowing yourself the flexibility of not being focused on producing a "set" number of words or pages will put you on the path to consistent writing. This is the key to success.