How do we get users to rate documents?

I run several document management systems for a large company. On the system most of the documents are word/pdf. There are some videos and html stuff but 80%+ is a old school document.

We are looking into search engine providers that help us reduce clutter. One of the ways good search solutions work is by using ratings and proximity ratings (recency and group being weighted). So we need people to rate documents!

However on 3 sites combined we average over 250k hits per month with over 20k downloads. We get about 10 ratings a month and if I examine the data a lot of the ratings are by people in groups that make the documents...

Right now we have a 5 star model that is right next to any document download on a screen. We run a basic google type search that lists about 10 docs per search result page and the 5 star just has to be clicked. Alas no one clicks it.

Probably the reason is most people download a series of documents (mostly sales or product related) to help them and would have no reason to ever visit the search results or that part of the site after they already have the information.

Really the only way I could get people to rate documents is to auto-send an email a day or week after download. However that was just too annoying to people so it didn't fly. What are some alternative methods we can use to get people to rate documents on the site?