If you think that when the App is completed and ready for the store you have done your job, I’m sorry but this is not true… A lot of work have to be done when the App is in the store.
No just promotion but continuous improvement also. More details? Check this presentation on slideshare.
Isola Software 
Another thing to know: there are issues with free software (GPL’ed) in the App Store. See http://www.fsf.org/news/2010-05-app-store-compliance and related pages.