When playing pvp, there is a thing called Darius Syndrome. It is when a player gets overwhelmed by Darius' power and gets all the kills. When this happens, you become target number 1. Now, when u have an enemy that u can kill but a teammate is already moving towards him, don't take the kill all the time, let them get the kill, this way you don't get ganked by the entire enemy team. Be sure to help your teammates, not take their kills.

Heal: I always have this just in case of someone doing large amounts of damage as i am leaving from a battle. This works great when you are less than 10% health. It also helps when fighting someone like Tryndamere, or Master Yi.
Teleport: Use in 5v5 matches. Darius isn't the fastest runner in the game, whoever has played him knows that, so this is a great way to save a turret from an enemy, and take them from surprise.
Flash: Used from running from someone with a lot of CC or a lot of movement, such as Nunu & Willump or Master Yi.
Exhaust: Also for running away or chasing someone too fast.
Ignite: To deal additional damage to your enemies.
Ghost: To escape or sick to your enemies.