Writers are mostly self-employed (excluding journalists and the like), so self-motivation can be a problem. There are a number of theories on the motivation in the workplace, but I’m going to look at the cognitive-choice theory.
According to this theory, there are three factors which affect motivation in work. Here they are:
This is the value of the outcome that the particular activity will give. If you work at a diner for a salary, the outcome is money in your hands. If you’re a paramedic, the outcome can be saving people’s lives. Every person attaches different values to different things (i.e. money can be very important for one fellow, but not at all important for another).
This is the possibility that the activity will lead to more desired outcomes. So if you’re a waiter at the diner, and you see that the manager is getting old, being a waiter as an additional outcome of giving you further beneficial outcomes (i.e. getting more money, working less). Or, if you’re an accountant, working as a clerk gives you experience which could open doors for you later, i.e. more desired outcomes springing from the first (in addition to your salary).
This is how attainable the outcome is. If you’re a freelance writer and a magazine offers you a lot of money to write a 8000 word article on a subject you’ve never heard of in two days, the attainability of the lot of money falls, and so does your motivation to accept the offer. If you are offered a job as an announcer and public speaking scares you, then it is not very attainable.
These three factors form a combined measurement of motivation. I.e. if the valence, instrumentality and the expectancy is high, you’ll go for it. If all of them are low, probably not. But when they have different values, say working on an oil rig, which gives you a lot of money but doesn’t have a lot of possibilities for future desired outcomes, each person will decide differently. Everyone assigns different values to different things. If the valence is high enough, some people might go for it even though the other two are close to naught.
When writing and having to motivate yourself, it might be helpful to look at these three things and ask yourself which is lacking (and then maybe you can push it up a bit).