In my experience at least, review building is something that's impossible to do without some pretty intimate cooperation from the client. The right time to ask for a review even is dictated by the processes the business has in place, and there's a lot of options depending on what works for the business owner/sales staff. If it's blue collar without a lot of tech in between the customer and the owner, you could even give them an easy way to verbally direct someone to leave a review. For example, after asking for feedback, if the customer's happy, you say something like 'we're glad to hear that, and we'd really appreciate it if you take a few minutes to write that down so other people can find us too. Just go on our website, scroll to the bottom, and look for the 'leave a review' link up top' or something like that.
You can either have that website link just forward to a pleper Google review, or Yelp, or whatever else you want. obviously if you do interact through email or whatever else, it's easy to send a link that way too. You get the idea.
I hear you though, it can be tough with certain clients and industries, and that's why it's important to find options that they'll actually use. All the tools in the world won't work if the client never makes it a part of the business.