Here's a neat trick. I've recently been playing, and enjoying, the online hitman role-playing game gpsAssassin from developer Side Hobby.
It's in the mode of Facebook and MySpace games such as Mafia Wars and Mobsters, but with a locational element, based on your iPhone's GPS or iPod touch's wireless connection/manual location.
I've just received by email this inducement to encourage my feedback via an iTune review.
"Did you know? You can send proof of a 5-Star iTunes Review to joy [at] gpsAssassin.com for $35,000 Blood Money (include your Alias). You can do this for every new version of the game!"
Note: Blood Money is the in-game currency used for buying weapons and armour.
Presumably this sort of thing if frowned upon by Apple but not prohibited.
However on the UK store, the game only has a 2.5 star rating (from 298 reviews), while on the US 2282 reviews give an average of the same rating.
Maybe the amount of Blood Money needs to be increased?