Finish the job visibly
Walk the completed work with the homeowner before the crew leaves. Point out what was done, what to expect over the first few weeks, and how to reach you if anything looks off.
A walkthrough converts relief into satisfaction, and satisfaction is what people write about.
Ask while you are standing there
The best time to ask is immediately after the walkthrough, in person, while the work is fresh. Keep it short and specific: ask whether they would be willing to leave a short review describing the job and how the crew handled it.
Follow with a text containing the direct link before you drive away. Asking in person and sending the link on the spot is the entire technique.
Make it easy to write
Blank boxes cause writer's block. Suggest two or three things they might mention: the type of job, whether the schedule held, how the site was left. Never write the review for them or offer anything in exchange.
A review that names the job type is far more useful to the next homeowner than a generic compliment.
Handle the unhappy ones properly
If a customer is not satisfied, do not ask for a review — ask what needs fixing, then fix it. Many of the strongest reviews come from jobs that had a problem the contractor resolved without argument.
Respond publicly to every review, positive or not, in a calm and factual tone. Future homeowners read the responses as closely as the reviews.