Making the process as simple and intuitive as possible is the key to getting people to do a task that would clearly have little benefit to themselves.
This is a very important concept, not only in getting reviews written, but in usability in general.
If you have data on your customers, perhaps you know who's on Twitter, Facebook, etc then reach out in way that doesn't require logins or registrations, send them emails that have URL's that expire after a set period of time so they don't need to login.
Then when you think you've optimized the process, run through it, have friends and family go through the steps - this is called usability testing. Companies with huge budgets will spend 10's to 100's of thousands of dollars to have people come sit in a lab and bang on an application while they take notes and tweak the design, speed, and other things that will improve success metrics.
Find out what a review is worth to your business. Since traffic from review sites can be tracked, whats the conversion rate, and then calculate the cost per conversion. This would come from the amount invested in the process, divided by the number of conversions. If it took $1000 of optimization but net another 100 conversions than it was $10/conversion. Then for every 100 conversions you get a sale, you can do a quick cost benefit analysis to see if the ROI warrants the investment.
