Log in daily, even if you do not plan on playing, to get some bonus Kiwi with a Kiwi Chest.
Missions
Completing the three missions you get each time will increase your level, as well as boost your multiplayer bonus by 1%. You will also get one free gem.
Power ups
Available power-ups include the Shield, Stamina Up, Kiwi Doubler, as well as Last Spurt and First Spurt (give 125 free meters when dying or starting).
Stamina
Stamina is used when flying and dashing. Avoid jumping until it is necessary. When trying to improve your score, it is better to dash against an enemy.