Effective user-experience writing is really problem solving. Virtually every word choice has a ripple effect. Add something in one spot, and it needs to be accounted for in several others, either via addition or subtraction. My approach is pretty straightforward. Write something as clearly possible. Then keep subtracting words and rearranging them until it's as short as possible while retaining the same amount of clarity. Once I get to the point where subtractions reduce clarity, I stop there and add words back in until I've hit the sweet spot. Then I test relentlessly in usability testing to get outside of my head and into the user's.