Gems
Collect as many gems as possible, the in-game currency. When a run ends, you will be given a chance to watch a video advertisement to earn more gems.
Mind the Shadow
As you progress, it can be difficult to tell where you can safely tap in order to keep things going. Watch for the ball's shadow, though, and you'll be fine. Make sure to tap while the shadow is positioned as close to the middle of the platform as possible, so that any minor timing errors don't morph into disasters.
