Getting your clients to review you in Google+ is hard at best. The Google+ Review Handout Generator makes this a lot easier. I recommend you go give it a try for your business. I am recommending it to all my customers!
In a nutshell, you enter your basic contact information and it creates a set of printable instructions (in .pdf format you can share with your clients) that walks them through, step-by-step, how to leave a review on your Google+ Places for Business (AKA Google Maps, Google Places, GooglePlus) account. Here is what mine looked like when I created it: