As a small indie dev (dealing with hundreds or thousands, not hundreds of thousands of customers), the best way I've found to encourage reviews is to prominently display my email address for any questions or ideas. Then, when customers write (many of them do), I write back a nice response and then add: P.S. If you enjoy Mindscope, would you take a minute to write a short review? It would mean a lot and helps give Min…
Some people here would probably call that a "dark pattern" (I of course don't). Because, you know, it's soliciting for positive reviews from a preselected set of users.
I don't know why people write AppStore reviews though. If I like an app, I'll go on my computer and tell people via social networking. Typing out a credible-sounding review on my phone sounds like work, not fun.