Jarven is a very powerful character, if used correctly. He jumps into fights, finishing off any wounded champions, which is how i like to play. I jump into (primarily team fights) and go for the weakened or just weak champions first. Cataclysm is a perfect skill to get in close distance of those pesky ranged buggers.

Ghost and Clarity will get you into or out of the fight with a full mana bar, especially if you spam the skills, Dragon Strike is especially useful to spam when you dont have a ranged to help you keep champions away while you farm.