I'm from the school of thought that your science isn't really done until you've told someone about it. Communication is essential to what we do, because we are supposed to be building a body of knowledge for the community, not just learning stuff to make ourselves feel smart. I take that part of my role and my education seriously, so please act surprised to know that I flubbed this question in an interview, "Do you have experience writing for a non-science audience?" Forcing my spouse to proofread my dissertation wasn't really the answer they were hoping for, but I didn't have anything more concrete than that.
At one time, I thought pretty seriously about developing a writing portfolio, or at least a few showpieces that could be collected should the need arise. I submitted some short things to writing competitions, and sweet talked my way into getting a brief article into a professional society newsletter. And then all this seemed like a lot of work, and I never quite got whole portfolio together. At this stage, I am not sure if a full blown portfolio would ever be looked at, but I am still working on my non-technical writing. If asked the same interview question again, I would point to this blog. But the benefit of writing regularly isn't that I've amassed of lot of Pulitzer Prize quality writing, but that writing regularly makes all my writing better. Right?
In some cases, I've seen applications that say outright that your cover letter will be used as a writing sample. You should certainly treat it as such. If you have non-peer reviewed publications, put it on your CV. If you are in a position where you interact with non-scientists, say, business people, or even just the HR person in charge of hiring you, they want to know that you communicate well in writing.
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In case you are interested, here are some interesting science-y writing contests. The SCRIPT Award for a Mini-Epic and the currently running Clerihew Contest, seeking short poems about science.