There's one consistent mistake I see non-junglers make while jungling nearly 100% of the time: inefficient pathing.
Let's assume you're blue side and want to gank bot. Plan your clear before you even leave fountain: Gromp, wolves, chickens, golems, gank? Chickens, golems, gank? Chickens, skuttle, gank? Whatever you choose, plan it before you go. Don't do that jungle shuffle, as Neace puts it, where you clear a camp then hesitate and walk back and forth wasting time.
The reason for this is simple: Jungling is all about power spikes, and abusing said power spikes, leading to effective ganks and leads that you can extend into further leads. If you clear before ganking, you'll level up, giving free damage/another skill that will make your gank more effective. Just get red buff? That's a power spike, albeit a temporary one. At level 5? strongly consider farming to 6 to get your ultimate before ganking. It's the little, efficient decisions that optimize your clears/ganks and lead to consistent wins.
The role is incredibly difficult to concisely explain in a single post, I'd watch a jungle stream if you want to really learn the ins and outs, just thought I'd share the #1 pitfall I see that tanks a jungler's chances of winning.