Briefing them so they can be concrete

Ask first, always, and never list somebody cold. Once they agree, send the job description, your CV exactly as submitted, and three or four lines reminding them of specific work you did together and what you would value them mentioning.

This is not putting words in their mouth. It is saving them from reconstructing a year of work from memory in the ninety seconds before a phone call. Referees consistently describe this as the most useful thing an applicant can do, and just as consistently report never receiving it.