We all have different motivations for writing. For some the promise of a paycheck is more than enough, while for others there must be something more nudging them along. I would like to guess that fiction writers have an easier time with this aspect of writing since they're generally doing work they love already, but I can't be sure since I don't write a lot of fiction.
I can say with some authority however, that writing non-fiction requires a good deal of motivation and something strong that will keep you writing day after day. Especially because most of the material that non-fiction writers have to deal with would not be their first choice of topic.
So how do you stay motivated to write every day even when you're writing about stuff you have zero interest in?
Find something about your topic that does speak to you.
In every article assignment there is the potential to uncover something you didn't already know. Doing research on a new topic, even a boring topic, can often give you a different view or some tidbit of interesting information that you can then use in your article. Even if you feel that you already have a great understanding of the particular topic, you can always find something new if you do a little research. So when in doubt or feeling unmotivated, research and read anything you can find that has to do with your topic. And don't forget to look at the counter-points for opposite viewpoints and information.
Imagine how you will feel when you finish the assignment.
I did this for almost every term paper I had to write in school, as well as some of the duller content writing assignments I took on to pay the bills while I was in school. Again, aside from the money I made writing these things (there's a reason a lot of boring writing pays very well, no one else wants to do it), I always focused on how great it would feel to get the project done and put it behind me. This method pushed me through everything from Accounting assignments to Military Law content, and at the end of it all I felt fantastic with what I had accomplished.
Promise yourself something when you finish the work.
While we don't want to go around rewarding ourselves with chocolate cake or anything, promising yourself something you really want is a great way to motivate yourself to finish a task you'd rather not do. Yes, this is a little like bribing your 3 year old to eat their veggies, but hey... whatever works right? Give yourself permission to do some gaming, go for a walk, or spend some time browsing in your favorite book store after you've fulfilled your duty. Promising yourself something like this also has the added benefit of instant gratification and reward for a job well done.
Ultimately, you have to find whatever it is that motivates you and make it work for you consistently. And yes, sometimes that might only be about the money, but do the leg-work before you fall back on that because you might surprise yourself with what you discover.