From Google Ventures: 5 Rules For Writing Great Interface Copy is a good article that discusses 5 good things to keep in mind when writing UI copy. I like “Headlines and buttons are the pillars of our products. See, there’s a good chance you’ve been writing product pillars this whole time, and didn’t even know it.
The author really nails and details some of the guidelines UI writers should practice daily (clarity, specificity, directness, labeling, etc.). His bit on personality is interesting in the light of how some UI lines and buttons need to be angled a certain way, like being more playful, while still being clear and logical. In such a situation, one needs to find a good medium in establishing personality/voice and getting to the point without being silly or confusing.
The key thing I think non-writers need to “get” from this article is that writing is part of the design process, essential as any other part, and is itself design. It has no single place in the design process, but needs to be considered throughout the process.