A few ideas:
- Charge a small subscription for saving scores etc, just take those options away from the free users or segregate it so you have the ghetto public stuff and the awesome subscribers' stuff
- add customization, ad-free and anything else you can think of that users might pay for ... yes there's an on-going cost but if you can get any dollars at all out of your biggest fans that's better than the penny or two they're worth per month right now.. a single $1 purchase a day almost doubles your revenue
- interstitial ads like mailarchis mentioned, find a way to put them in and show them to each user at least once per day
- get a referral system in place so that these people who obviously love your game are motivated to share it
- outsource the backend through companies like http://parse.com/ if you need a full-blown database for your app might be cheaper than your current arrangement, and we (http://playtomic.com/) are debuting our first Android version next week which has free leaderboards that might suit your requirements
- sell the ad space yourself or use it to promote other games of your own
- Charge a small subscription for saving scores etc, just take those options away from the free users or segregate it so you have the ghetto public stuff and the awesome subscribers' stuff
- add customization, ad-free and anything else you can think of that users might pay for ... yes there's an on-going cost but if you can get any dollars at all out of your biggest fans that's better than the penny or two they're worth per month right now.. a single $1 purchase a day almost doubles your revenue
- interstitial ads like mailarchis mentioned, find a way to put them in and show them to each user at least once per day
- get a referral system in place so that these people who obviously love your game are motivated to share it
- outsource the backend through companies like http://parse.com/ if you need a full-blown database for your app might be cheaper than your current arrangement, and we (http://playtomic.com/) are debuting our first Android version next week which has free leaderboards that might suit your requirements
- sell the ad space yourself or use it to promote other games of your own