To summerize it: traditional marketing like AdWords or banners a big Apple magazine websites does not really pay off. What currently works is trying to get as much PR as possible and nail down the Apple Staff to pick your app. I wonder though what data made Robert Marini conclude that most iPhone apps do not benefit from light or demo versions.
Financial Realities of the App Store
December 15th, 2008
08/08/2010 - Last.fm with Snowtape Phone 1.3
07/30/2010 - Announcing Jubilee 1.1
07/29/2010 - Optimizing Snowtape for Retina Display
07/23/2010 - Jubilee 1.0.1 - 50% off
06/30/2010 - Multitasking with Snowtape 1.2 for iPhone
06/01/2010 - Winners of a Jubilee Promo Code