Ask every satisfied customer, at the moment of satisfaction, with a direct link that opens the review box in one tap. The mechanics matter more than the message: friction, not unwillingness, is why happy customers don't review.
Build it into the job's ending. Text the link when work completes, same day, personally. Steady drip beats bursts: a review a week for a year outperforms twenty in one month (and looks organic because it is).
The bright line: never pay, incentivize, or gate ('only ask happy ones after screening'). All violate Google's policies and, for incentives, FTC rules on endorsements. Describing the rules, not lawyering: just ask honestly, every time, and you'll lap competitors who don't.