Another strategy--and I'm not sure about the morality of this, except it's less evil than the above--is to pop up a "please rate!" dialog right after doing something nice for the player.

For example, have a random time-based trigger that will unlock a DLC item in the middle of regular play. Tell them they got it, make it shine and sparkle, and then--maybe not right away, but after a minute or so, after they've gotten back to the game and might be playing around with their new toy--ask them for a rating.

Another way to put it is: if Nintendo, as some unknown up-and-comer, had released SMB3 on the App Store, a ratings dialogue would almost certainly appear approximately two minutes into the Kuribo-shoe level.