Use Charm to set up your combos, it will make landing Orb of Deception and Fox-Fire dramatically easier.
Initiate team fights using Charm, and chase down stragglers with Spirit Rush.
Spirit Rush enables Ahri's abilities, it opens up paths for Charm, helps double hitting with Orb of Deception, and closes to make use of Fox-Fire.
Ahri's survivability is dramatically reduced when her Ultimate, Spirit Rush, is down.
Stay behind minions to make Charm difficult to land, this will reduce Ahri's damage potential significantly.