Placing a Demacian Standard increases the stats of allies as well as one's own. Remember that Jarvan IV gains both his passive's and the standard's bonus stats.
Placing a Demacian Standard in a team fight can be very rewarding, as not only do you aid your allies in battle, but you also get more assists whenever one of your allies kills an enemy while having the Demacian Standard buff (even if you didn't deal damage to the enemy champion).
Jarvan IV can have a deceptively long nuke radius. Consider using your Demacian Standard and Dragon Strike combination to close distance on a target, and using Cataclysm immediately afterwards for the nuke. You could also add Flash to the combo to close the gap even further.
Using Cataclysm to cage multiple enemies is a great way of dealing damage to them without harming your own teammates. Be careful where and when you place it, as it may not turn out to be in your favor. Using Cataclysm at the wrong time/place can cause unnecessary death to you or your teammates, for example caging the wrong enemy out or caging low-health allies within.
Enemy champions such as Shaco, Ezreal, Shen, or other champions with a similar short-ranged teleport/dash ability (including Flash) can easily escape Cataclysm. Be aware of this when using Cataclysm on these targets.
Be careful when using Cataclysm on enemy champions that do high AoE damage to enemies around them, such as Amumu or Kennen, or in range of enemy champions that have ranged AoE abilities, such as Brand or Malzahar. Accidentally trapping your teammates with or within range of these types of champions will make it so they take full damage from otherwise avoidable AoE abilties.
If built to survive, Jarvan IV can start a team fight with great success. He also deals damage well without many items. While building him, strive to maintain both survivability and damage. A team with skills such as Bandage Toss can be used to get inside the Cataclysm and complement Jarvan IV's ability to initiate well.
Combining well-placed Demacian Standard with Dragon Strike can disrupt the enemy enough to turn the tide in a battle. This can also be used to chase an enemy or cut them off by pulling yourself across terrain.
Be wary, as Cataclysm is one of the few abilities in the game that can actually bring about the death of an ally, by trapping them at low health with an enemy or a turret.
For this reason, wait until his Cataclysm is on cooldown before engaging him.
The ring of Cataclysm can be escaped from by virtually every dash in the game. Having one off cooldown can quickly nullify the value of the ring.
Do not underestimate Jarvan's 1v1 potential. His damage output and his Cataclysm make him a dangerous fighter.
Be even more careful of him with his allies about. His abilities have a multitude of utility effects that can let his team win against yours easily even if you personally are able of fighting Jarvan's 1v1.
While Jarvan's knock-up is threatening, do remember that it relies on his Demacian Standard, which is on a 13 second cooldown. If you can get away from the flag, he won't be able to knock you up again.
Scatter around before engaging a team with Jarvan, it will make his ultimate less effective when he can only trap one person.
Take great notice of Jarvan's team. If it consists of heavy AoE (Morgana, Brand, Fiddlesticks, Malphite, etc.) then consider fighting the enemy on your grounds than theirs, or, if possible, eliminate Jarvan before the team fight starts to make their ultimates less effective.
Due to the great utility of Jarvan, he can participate in a "kill lane" (a lane where the aim is killing the enemy, generally two bruisers against a support/AD carry in bottom lane: popular duo is Jarvan + Leona). If you see such a setup, consider using Alistar or other non-ultimate disruptive supports to stop the kill lane's success. (based on personal experience).
Using a silence on Jarvan will stop him from dashing to you, or even casting his ultimate. You might want to save your silences for that. Even better, sometimes the ultimate is used to do damage and crumble the wall. A silence will prevent him from crumbling the wall, making quick work of him in a bad position.
Attempt and avoid basic attack harassment by Jarvan. Due to his passive, it will almost double his basic attack's damage in early levels, making you generally unable to pay it back and leading to low health.