Kiwi - League of Legends

Let me start this entry by stating that I'm no pro, but this is just a set of basic tips and errors I've seen so far, mostly for not paying attention and going into automatic mode while playing.

Farming
The main reason for jungling is having one extra solo, which is extremely useful, but jungling means more than do a first rotation and then just gank like crazy, or struggle to keep up with bot lane in levels.
The jungler needs farm too, farming is really important, in every role (except maybe support), you can't just skip wraiths and golems to gank top, or else you will get behind (of course there are times you have to, but let experience decide that), you have to farm, gank according to your jungle path, if you wanna start with a gank bot, you can start at blue and gank at 2, or if you want to gank bot at lvl 4 you can start at golems, do wraiths, wolves and blue and then gank, if you want top at 2, you should start at golems, the point is, gank when you are passing by, that way you will be able to farm nicely, the jungler is important too!
Keep your jungle clean all the time.

Do not push
Covering solo lanes normally gives more experience and gold than jungling, do this when you can, but remember you normally do not want to push the lane, it's a very common mistake in low Elo to just cover the lane and push it completely, or just after ganking, the jungler pushes the lane for some reason, sometimes this is good, if you ganked bot and killed them both you want to push the tower, but if you ganked top, and just made him flash, dont push!

Ganking
Ganking is really important, is what most people think jungling is all about, this is true to some extent, but it's also very risky, a jungler who has a lot of failed ganks will mostly get behind in mid game, be underleveled and underfarmed, you can't help every lane, you will get blamed if you dont gank, or if you gank too much, just ignore and farm, gank when your jungle path allows it, and if there is a chance to kill him, or at least make him burn his summoners.
People normally ask for ganks when they are losing the lane, this is tricky, because if your partner is at 30% health and the enemy is pushing, you might have a chance to kill him, but there is a chance he kills your teammate too, and that's not good, and if the lane is 2v2 and they are losing pretty badly, there isn't really much you can do, you want to really gank when your teammate is winning, or when it's pretty even, in that situation you will have a way higher chance of killing your enemy. Of course there are exceptions, sometimes they overextend way too much, or you are fed, or whatever, but playing safe is always nice.
Another thing is don't sit in the bush for five minutes waiting for them to overextend, if you notice they were pretty aggresive and then they b when you get close, they saw you, just keep jungling, if you failed your gank, you can try again but don't stay there too much, is not good for you nor the lane.
Finally, some people wont even see you coming, you should always type "Im ganking top in a sec" or something like that, so they know not to die, push, nor do anything stupid, and they know you will go, also, ping when you are ready, and if it's a dual lane, ping who to focus.

Do not use the river
Avoid using the river, the river is always warded, they will see you, and you will fail, using the river is only good at lvl 3-4 when it's not warded yet, but after that it's even better to just walk by the lane and gank, be a bit creative with that, there are some places you can use, like, if your bot or top lane are pushing, you can sneak in the bush without beeing noticed, you just need your teammate to initiate to do a nice gank, if they are getting pushed, it's the same, just walk in the jungle close to the tower, and let them initiate, if you can initiate, you should do it though, champs like Warwick and Nocturne are good at that (Nocturne is specially nice for ganking, he can be practically undetected by wards).

Counter Jungling
Counter jungling is basically when you take the enemy jungle, or they take yours, if they take yours at lower levels (their whole team stole your wraiths and golems), there isn't much you can really do, luckly this doesn't really hurt much (you should have a backup jungle route), but if they keep taking your jungle, you should take some actions, the normal thing would be take their jungle, you can also ward and try to gank them when they are there, this isn't that common after level one, and it's really risky, but there are some champions really good at this, like Shaco, you can counter jungle really safely, even try to take their fist red and things like that, but have in mind it's risky and you can get behind if you CJ too much.

Know where their jungler starts
If you know where they start you can CJ safely, for example, you jungle against a jungler you know they start at blue, you can then do your blue, then go to their jungle and take their wraiths or their golems, as after blue they will probably do wolves, also, you can get the timers, for a standard blue start route blue will respawn at 7:10, red depends a bit on the champ, Udyr for example clears the first rotation super fast, warwick takes a bit more.

Get the timers
It's 5 minutes for buffs, 6 for dragon and 7 for baron. Time your own buffs and you enemy's, control your jungle, be there a few seconds before blue spawns, so they can't steal it, same with red, and of course, control dragon. Jungling is rather easy, you have enough time to see the map, think, see where to gank, when to do dragon, etc, control your jungle!
Typing in chat the timers help, if not for blue and red, do it for dragon and baron.

Have several possible routes
Many new junglers have no idea how to jungle when they get blue stolen, or wraiths and golems, to be a solid jungler you need several paths, this isn't really hard, and sometimes includes some CJ, this is just something to have in mind.

Pay attention
Jungling is not like laning, you just right click and maybe use some abilities, but you have time to see the map, if you are doing blue and see the enemy going mia, they might be comming to you, you should stop doing your blue and go away, maybe stay around and try to steal it, but pay attention to those things.
Also, it's not really necessary to smite the creep to kill it, if you are sure where everyone is, and there is no danger of getting it stolen, personally I dont really care much about that, but everytime I do blue o red I do it in the bush so I'm sure there's nobody there, it's a nice practice to do.

Get blamed
You will get blamed by everyone who lost his lane or got ganked, just ignore them, dont die and keep farming.