How can one ensure that the style guide is maintained and used?
I'm specifically working with UX styleguides.
The best way to keep a style guide up to date and widely used in the organisation is to make it as a Living style guide, following a Styleguide Driven Development approach in the product development process.
Here are a few definitions of those two terms:
Living style guide definitions:
“A design showcase built using live presentation codebase.”
— Chris LoPresto at EmberConf 2016 ▶️ [7'00"]
“A reference for a team of designers and developers to understand how an application will look and feel.”
— Patrick Hildebrandt in What Is a Living Style Guide?
“A living document of code, which details all the various elements and coded modules of your site or application.”
— Susan Robertson in Creating Style Guides
“A live version of a brand style guide.”
“Living style guides are something that should be developed with every project”
— Brian Eye in Night of the Living Style Guide
“An important tool for web development today, especially in large, complex web applications.”
— Steven Lambert in Creating A Living Style Guide: A Case Study
“A documentation of UI elements and patterns collected from a site or application with the purpose to allow developers to use consistent styles across their whole project.”
— Nancy Young in Ten Living Style Guide Tools for Web Designers – Best of
Styleguide Driven Development definitions:
“A practice that encourages the separation of UX, Design & Frontend from Backend concerns. This is achieved by developing the UI separately in a styleguide.”
— Jordan Lewis in We build Envato: Styleguide Driven Development
If you need to go deeper on how to implement a Styleguide Driven Development process in your organisation, a simple search on YouTube will bring you a few good presentations.
To finish, a very good example of style guide that implements these concepts is the lonely planet style guide. They even wrote a very good article about why and how they built it: A Maintainable Style Guide
To be maintained a style guide needs to be online as a blog or a wiki. To be used a style guide must be based on principles rather than on rules to be a guide and not a break.