I just play League for fun with friends nowadays, but I can tell you the 5 things I tell anyone picking up the jungle, or struggling with it.
1. Play by the tier list.
Follow the tier list on nerfplz.com (just Google "tier list"). It updates frequently, every few weeks, based on patches and the constantly changing meta. Playing by this list will always garner you a higher win rate simply because you're playing by statistics. Think of sports; teams pick up players based on their statistical performance, not just because they look good or have a certain style.
2. Try to avoid team-reliant junglers.
Who are some of the best junglers in solo queue right now? Udyr, Master Yi, and Xin Zhao. Not Amumu, not Malphite, not Skarner, and other champions who you would think have amazing engagement tools for the team. This is solo queue, you can't expect that your team will be able to capitalize on an amazing play.
3. Map awareness.
You need to keep an eye on all lanes at all times. Ping missing enemies if a laner forgets. Ping to remind teammates that they're overextending. Ping to warn teammates that someone just passed through their ward, or is on their way to the lane. You also need to always know where the other jungler is. This comes largely with experience - knowing routes, clear speeds, and travel speeds. Paying attention to where the opposite jungler is will open up more opportunities for safe Dragons, ganks, pushes, camp steals, and most importantly - counter ganks.
4. Do not waste time.
This is probably the most overlooked concept. I see players above Gold still making this mistake. Do not spend more than a 2 or 3 seconds waiting for a gank. If you don't find an opportunity either leave or reveal yourself to apply some pressure, then move on. Likewise, you should never be changing directions abruptly, or wandering the jungle. Move from camp to camp to lane, and never stop moving. Every second you waste is a second you could've been clearing a camp, pushing a lane, ganking, or counter-jungling.
5. Don't waste opportunities.
Another big mistake I see junglers making. They gank a lane successfully, then go back to jungling. This is the best time to push a lane or do Dragon, because their team cannot possibly create a numbers advantage. Unless your team is low, the enemy is likely unable to contest.
