Disclaimer: I am not in any way an expert.
Your main options (besides moving to the paid app model) are in app purchases and advertisements:
In app purchases pros:
No advertisements clogging up your screen, meaning improved user experience and interface.
Infinitely scaleable to your imagination, just look at Zynga, whereas advertisements are limited within the context of an app.
If your app is addictive these can be very lucrative.
In app purchases cons:
Requires much extra content creation, or limiting or the initial app.
Most users don't use them, meaning the majority of your user base will be losing you money.
Can come across as cheap and that your paying users are just "buying" their success, especially if it takes off.
Advertisement pros:
Much more distributed income generation, you aren't relying on a select few purchasers.
A spike in traffic will generally directly correlate to a spike in income, not always true with in app purchases.
Much less difficult to manage once it has been set up, very much a fire and forget weapon.
Advertisement cons:
Most users become annoyed by pop up windows and adverts taking up half their screen. - This is a biggy.
The ad agency takes the lions share of the profits, so you could potentially by capping your income to a certain amount when in app purchases could be more lucrative.
