Be willing to try new things. Go outside your comfort zone to make gains.
It's amazing to me how many of these answers apply to my life in general at this moment. I feel like I am at a place where I need to make changes and find my place. All of these quotes speak to me in that sense, but back to writing. I find that if I am "stuck" on a project, in any stage, it helps me to look at it in a new way. Maybe I need to try a different way of plotting my story out (maybe even something as simple as using a drawing rather than a list, etc.), maybe I need to try to look more closely at what my characters are doing to push their own boundaries.
A good story involves some kind of change to the main character. Maybe they have some amazing self-discovery, or their experiences change the way they look at the world, or perhaps they are physically changed. Whatever the case, they are required to move outside of what is familiar and comfortable, and into the unknown in order to grow. Just as we have to push ourselves as writers to always reach for the next level, try the next new thing.
So go out and make yourself uncomfortable today (grins), it might be a lot of fun.