The best time to ask for a review is once you have successfully helped your customers, I see a lot of sites ( especially eCommerce / Service related ) get a good review if they can email the customer and ask for one immediately after a service or product is purchased. ( assuming they had a good experience )
You can easily generate a direct link that you can embed in your email using this tool . ( NOTE: I was involved in making that )
I don't believe you can leave a review on Google anonymously. But you can create a review page on your website where you can publish these reviews. Bu sure to markup the review to stand out in the SERPs.