Ask, and you shall receive
Ask them to submit reviews for what they bought during their previous visit.
You could have a section high up on the homepage, and/or a notice in the header, or a popup. It should be highlighted also in the Account Dashboard.
One of the members from my Ecom Convert group added some thoughts:
Check at login, if the registered customer had purchased products and if the said products have not been reviewed, then present them with a page that asks them to review the products they purchased. This would be an account page, if there is more than one review required. If it is only one product, then the actual Write a Review page would load for that product with a message at the top saying we noticed you have not yet reviewed this product, so git ‘er done!