One more idea I gleaned from tim ferris' blog, I really should keep track of these:
* Make a freemium app, with an IAP to upgrade
* Freemium means it's easy to get people 'in the door' so to speak
* Once they're 'in the door', pop up a nag alert every now and again (eg every 5th launch) to suggest why they should upgrade to the full version
* Each nag alert, recommend a different benefit of the full version, so gradually it'll convince them to purchase.
Sounds reasonable to me. It all seems to be about having a 'smooth funnel' - eg on your app's promo site, don't try to convince them to buy it, make it a smaller step: ask for their email instead. Make the app free, so it's a small step from finding it to installing it. And once they're in the app, gradually convince them into purchasing the full version with 'nag' alerts.
Of course, nag politely, don't put people off ;)