As a writer in both creative and technical fields, I've contributed to my share of user-centered copywriting, from onboarding to email messaging to action labels, as well as menus and dialogue systems in video games.
It may not have been an explicit part of my job descriptions, but plenty of these projects were in need of a consistent voice with the end user in mind, and I love bringing that voice to my work.
Bite-sized info, simple questions, helpful repetition
Whenever possible, I keep things concise and easy to understand at a glance, keeping non-native English speakers in mind. I've taught English to children and EFL students since high school, and trimming out colloquial speech and idioms really helps with comprehension.
I also like to repeat words between questions and possible answers, strengthening the connection between the two. "Yes" and "No" are fine, but repeating key words from each prompt make users more confident with the responses they've chosen.
Writing for email
Messaging in emails and notifications can be a difficult task. It's tempting to be more generous with your words when you've got all the space in the world, but brevity's just as useful here, especially as people tend to get more emails than they can realistically read during the day.
Emphasizing purpose and actions
Buttons and links should be as clear as possible about their actions. Bland, all-purpose labels like "Done" and "Submit" say nothing about the action behind them.
Of course there are no unbreakable rules, but I haven't really been convinced these words alone ever portray the actions behind them, although in some cases, "Submit my information" may be appropriate.
When onboarding and instructing users, it's important to know the purpose behind each action, so they can recall the action when it's needed later. Highlighting certain words with variations in text size, color and style can also help with comprehension and recollection.
UX writing presentation
I've got a presentation in progress that explains my approach to UX writing, and I've given it to a couple small groups so far. I'm adding more to it and plan on making it available online some day soon.