I have a donation button on some of my free apps, i just only received 1 donation from an user in more than 3 years, then i offered an InAppPurchase that gives: unlockable premium content + no ads, i received 9999% more revenue with this strategy.
I think we also need to know how to sell or generate profit with our work. you can offer a free app, we can give the user the posibility to have more features, if the user likes your app, will like more if he can have more PREMIUM or EXTRA content/features for a small amount.
Sorry my bad english.
Regards.