If you run out of lives and you're in a hurry, close the app and take the following steps.

1. Access the Settings menu.
2. Choose Date and Time.
3. Set the time ahead a few hours.
4. Open the Farm Heroes Saga app again.

You should find that your supply of lives has been magically replenished. You can repeat this trick as often as you like, provided you don't mind changing your device's time setting frequently.