The Demacian Standard and You

Jarvan IV's E is one of the more unfairly neglected abilities in League of Legends. While most players see it as a vehicle for Jarvan's EQ knockup combo, neglecting its other traits and potentials is a huge waste.

Sight

Jarvan's flag grants sight, and should generally be taken at level 1 for safe bush checking.

Statistics

Jarvan's flag grants Armor and Attack Speed to every allied champion in range. You notice that little blue shield in front of Jarvan? That appears on every champion affected by the flag's buffs. Whenever taking a tower, or Dragon, or Baron, Jarvan should throw his flag somewhere such that his team gets the Armor and Attack Speed buffs, but that Jarvan can use to jump on the enemy (with Q) if they appear and try to engage.

Niche Uses

As can be seen here, the Demacian Standard is a valid 'ally' target for Safeguard, Leap Strike, and Shunpo. Note that it is NOT a valid target for enemy-targeted jumps such as Cutthroat or Audacious Charge.

Teleport, just like Safeguard, can be targeted to an allied Jarvan flag, including your own. 

There's no reason to do that last one.

When a teammate (even you!) picks Jarvan IV, think about these things. Your team might have some possibilities that hadn't occurred to you before. If you are playing Lulu, use your Jarvan's flag to siege turrets with Glitterlance, or throw the Shard of True Ice effect onto the flag to prevent the enemy from defending the tower. You may just find out that Jarvan is a greater asset to your team than expected.