Something I think we don't discuss often enough as writers is diversification.
In this case, I don't mean diversity of subject matter or characters, although those things are important, as well. Rather, what I mean is that we ourselves must be diverse.
Think about it: in nature, stagnation can--and often does--mean death. If an ecosystem isn't varied enough, it can fail, withering away as its few plants and animals falter under the weight of sustaining something they aren't equipped to handle. Likewise, going hard on one project, all the time, can mean creative death.
It's important to branch out into other areas. Of course, writers should always be reading, and that's one way to diversify: by seeking out genres and topics outside of your own scope. And in order to keep the creative juices flowing, it's helpful to have another artistic endeavor. For me, it's photography (I'm a big fan of Instagram). When I don't feel able to write, or when I need a break from it, I might snap a few pictures and express myself that way. Other options are music, painting, or even crafting (my crafts of choice are usually crochet and scrapbooking).
Or maybe you have a more scientific hobby. Vladimir Nabokov was into collecting butterflies. Flannery O'Connor raised peacocks. Sylvia Plath kept bees for a time. Hell, even the more run-of-the-mill things out there, like cooking or bowling, make great side pursuits.
The point is, having something to do outside your writing time makes you a more well-rounded person, which in turn makes you a better artist. Sitting at your desk all the time can be crazy-making and lead to myopia. And no one likes a writer who never introduces a new subject or element to his or her writing, right?