Some people at Paizo emphasized they would like to see more reviews. Especially since the latest products got even less ratings than those before, I collected some ideas which might help:
Options with (relatively) little effort
1) Make the "write a review" link more prominent. I wrote several reviews in the past, but I still have to search for the link every other time.
2) Encourage people with a sentence that you want reviews. From the discussion I remember several people were hesitant to write, even if they could make up valuable contributions (from my point of view).
Option with more effort, likely
Protect the review text from being killed by the need of another login. If you spend enough time to write more than a few sentences and click "Submit", the page requires you to login again - and the text is gone. The frustration will ensure many people won't write such a long review again. Sure, there are ways around it (saving the text with Ctrl-C, for example), but not everyone knows about them.