It has to be said that the current iTunes system for rating apps has a certain element of bias – the current methods means that probably the majority of people who are leaving a rating are doing so when they delete an application. However the code on the following page allows you to remind users from your application at set time intervals to leave you a review. Hopefully encouraging a more positive review bias.
Thanks to Arash for making this available in a simple package for integrating with your project
http://arashpayan.com/blog/index.php/2009/09/07/presenting-appirater/