It is, however, necessary to fight through that brief "down."
Every writer has this problem, and every writer deals with it differently, yet here are a couple of tips that I personally find very helpful. It might help you along the way as well!
1) Write (about) something different.
I often find myself staring at my computer screen or getting frustrated over not finding the perfect words. Sometimes, however, it is necessary to acknowledge that fighting with the same strategies will not always work. If you apply the same principles, the same problems will pop up again! If you try something entirely new, whether this means throwing the old manuscript out the window, giving it a twist, or momentarily focusing your attention on a new (practice) story, certainly try it out! A new approach will bring freshness to the story. Furthermore, "writing something different" does not even have to be content-related. Maybe you just need to write in a different style, genre, or with a different voice. The more you try, the more smoothly the words will come to you later.
2) Change your environment or approach.
Changing your setting or writing area can entirely change your writing and consequently spice things up. Moreover, it might get you unstuck. If, for example, you usually write in a quiet room, try going out to a café and writing there. You might get entirely different ideas and inspirations, and, on top of this, it will enable you to observe the people around you. I sometimes also choose to write by hand rather than my usual typing, and this sometimes helps me produce ideas and keep going.
3) Write in a different order.
Usually I start my story with either an ending or beginning in mind. I then fill in the pieces in between. This, however, is simply one way of doing things. You could just as well know the plot and then create a start and end, or you could start with a mere scene in mind. If there is a scene that is currently vivid in your head, write it first before the idea gets lost by the time you get to it!
4) Read your favorite book(s).
Read books you like that inspire you because, quite simply... they inspire you. Ask yourself why you like the book(s) and apply it to your own work. Wouldn't you want to have the same effect on other that that author has on you?
If you currently have no inspiration, no sweat. Focus on other things and experience life. Later come back to your writing and view it with new ideas. You'll have new ideas and new motivation. Nobody feels inspired all the time (for me it is often not there at all, but when it is, I cannot stop writing for days and days on end). If you don't feel like you can take a break from your writing or you have a deadline to make, then simply put yourself to writing something each night (or morning - whenever you usually write). This will put you in the right mindset and make you more fluent and time-efficient in the future. (You could even try timed exercises to further train and challenge your brain.)
** This post was recently guest-posted on Shannon A. Thompson's blog :)