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.
Here's a particularly shameless example: http://imgur.com/8UGRals
But yeah, you'd probably get better results if you're like "hey, here's this free thing, want it?" (tap YES) "Here you go!" ... "Please go rate us thanks it really helps" because of people's sense of reciprocity.
In any case, my point is to use your fans and your customers to boost your rank, rather than shady services and botnets.