If you could only give one piece of advice for improving a resume, what would it be?

What is your go-to advice when someone is trying to improve their CV/resume?

Ask someone to review it.
Seems obvious but someone who is not a family member or a friend but is or has been a manager in the career field you are in/getting into.
They have a lot of insight into what expectations are.
Other chunks of advice:
Look at what is posted in JDs to see how you align with the jobs you want. You could be missing something obvious or have a requirements gap.
Ask a recruiter how they typically process resumes.

Condense it down to no more than 2 pages.

Given the amount of CVs an employer receives for each role, they do not have time to read through pages of non-relevant content. Get the most important qualifications, experience and expertise on to the front page with an easy to read format / style.

Keep it simple and aligned with the job your are applying for.