Hey, congrats on the success ! I would suggest focusing on retention, social sharing and ratings to keep yourself in a good app store positon.

Retention : add push notifications, try to stay in touch with the users (with limited offers for example as mentioned below) and to make the user come back to the app

Social sharing : I would suggest creating a leaderboard, and connecting to FB to share your results and compete against others.

Ratings : Implement an incentive to let the user rate your app, to grow the number of reviews you have.

PS : I didn't have time to download the app, so maybe you already did this. I will download it in a couple of hours and will edit. If you want to chat more, alexei [dot] chemenda [at] gmail [dot] com !