Since the 5.6 patch, Cinderhulk now gives 25% bonus health. No less. This greatly affected the meta, especially in toplane where some champions now play with a smite to build a Cinderhulk, but this is a great buff for tanks overall.
But why Shen jungle?
When we look at this champion, we notice that he has an excellent synergy with the Cinderhulk, which basically gives him 25% extra damage every 9 second thanks to his passive.
But this still does not answer our question! Why not Shen top with a smite?
Of course this is possible, and in the current meta. Shen jungle is not in the meta, I know it and I just play it for fun.
But there are a few advantages to play him in the jungle :
First, he has great sustain. I was surprised to see how little HP he actually loses when he jungles. This means that you can virtually gank at full life. It is practically impossible to fail a gank with Shen and to give a double kill to the enemy's laner.
Also, Shen has a good dueling potential, which is important for a jungler, as you may have to face invades. Moreover, as a jungler, you will sometimes arrive too late on a lane, and your ally may be already dead. In that case, Shen is particularly good to catch greedy ennemies thanks to his Shadow Dash(E) since you can pass through minions.
The last think I would like to mention is Shen's ultimate ; when you use your ulti as Shen top to help another lane, this can result in the loss of your tower, especially if you play against fast pushers like Tryndamere, Fiora or Master Yi. If you play as a jungler, you will not have this problem ; of course the ennemy jungler can go in your jungle to steal your farm but this is not as concerning for you and your team as the loss of a turret.

Pros:
Great sustain ; Shen loses little HP in his jungle.
Good dueling potential, which can catch some people off guard.
Decent ganking potential thanks to his Shadow Dash(E).
Decent damage output for a tank thanks to his passive Ki Strike.
His ulti Stand United provides him a good map presence and can save your allies from terrible situations.
Great engage in teamfight with Shadow Dash(E).
Shen uses energy instead of mana so he does not need the blue buff.
You are a ninja.

Cons:
Slow clearing speed.
No split push potential(with this build).
Poor counter-jungler.
Shadow Dash(E), which is your only escape, costs half of your energy and has a high cooldown so use it wisely.
No cc except Shadow Dash(E).
Stand United(R) also has a high cooldown, even though it has recently been buffed.
You cannot carry alone with Shen, you totally depend on the follow-up of your teammates.(If you E in and your team do not follow you, you will probaly be quite disappointed...)

Always rush Skirmisher's Saber with Cinderhulk, as this is what makes Shen really strong in my opinion. Cinderhulk helps you to clear faster and synergizes well with Shen's passive Ki Strike. Also, it will make you insanely tanky in late game, like most junglers using this item.
Then I suggest you buy Boots of Speed, as it will help you roaming to help your allies and Giant Belt since this item will give you the tankiness you need for early ganks and also a bit of damage.
You can also replace Giant Belt by a Sighstone if your team needs more vision (or if you support does not put any ward on the map...). This may help you foresee the enemy's ganks, which will always helps your laners, or simply kniw where the enemy jungler his, which will always help you. Read the Sightstone notes in the Items section for more information.
Moreover, this item is also great for its versatility ; it may be upgraded in three different item that are all good on Shen depending on the situation namely Randuin's Omen, Warmog's Armor and Frozen Mallet.
As for the defensive items, you should always build them according to the enemy's team ; versus heavy AD go for Randuin and Thornmail, versus heavy AP build Spirit Visage and Banshee. In any case, you should always build a Warmog whenever you can because it has an excellent synergy with your passive and with the Cinderhulk.

You can start wherever you want thanks to your sustain but I suggest you start bot side to get a leash from your botlane.
Always let your blue buffs to your midlaner unless he feeds too much(except the very first one of course : you need farm and XP). Shen does not need the blue buff whatsoever, there are plenty of champions who can make a much better use of it.
You farm slowly but you do not lose much HP ; take advantage of this to countergank junglers that do not have as much sustain as you.
In mid/late game, you should always let your red buff to your adc, especially if he is fed.
The slow it gives on your auto-attacks is great for early ganks, since you have only one cc, but once the laning phase is over, you no longer need it.
As a jungler you should always look out for farm on the lanes ; this does not mean that you should steal farm from your allies gratuitously but that you should always come to hold their lanes whenever they need it.
Shen is also pretty good for objective control, your Q gives a little sustain to your allies for Drake, your passive proc on turrets so you push fast for a tank.
Since Shen clears slowly, he has a poor counter-jungling potential ; however, if you notice that the enemy jungler is on the other side of the map, do not hesitate to do one of his camps and to leave a few wards behind you. Be careful though, never do this if you do not know where the other enemies are ; you will just get trapped and probably killed.

You surely do not have the best ganking potential in the game, your only cc being your Shadow Dash(E) and your damage output being rather low. However, the 1.5 second taunt can suffice for one of your ally to burst the target which you taunted. That is why I think that Shen synergizes well with champions that can deal a lot of damage in a very short period of time like Ahri or Diana for instance.
Anyway, unless your laner has a lot of cc like Maokai for example, you will have a hard time ganking champions with reliable escapes like Jax, Le Blanc or Zed.

Here are a few tips for ganking :
-Try always to gank the enemy from behind, so that you can save your Shadow Dash(E) for catching the enemy when he tries to escape. After you used your Shadow Dash(E), if your laner had not the damage to kill his opponent it is no use staying on the lane (unless your target has 20 HP, in which case you just have to flash-Q or flash-auto).
-Your ultimate is a great ability to countergank from the other side of the map. This can turn round 1 vs 2 situation in your favour. If your ally is really too low to fight back, this is not a problem since you will have saved him, which is already satisfying enough, and in addition you will be here to hold his lane afterward.
-If you are blue side, after your golems and red buff, you can smite your raptors to try to gank the midlane. Do this only if the enemy midlaner has pushed his lane significantly. If you go for an early gank, you must of course unlock your Shadow Dash(E) at level 3. But do not take your E right after you finished your raptors, wait to see if the enemy midlaner had warded before. If he had, do not take your Shadow Dash(E) at level 3 ; put another point in your Vorpal Blade(Q) instead and go back farming. If this was not warded, now you can take your Shadow Dash(E) and try to gank. Those kind of early ganks are really good, as the enemy midlaner will not be expecting it, nor the enemy jungler who should not be able to countergank you. Do not hesitate to use your flash if you know that you can force their midlaner to flash, or better still, if you know that this will secure a kill. In any case, it is worth, since it will greatly help your own midlaner.

As a tank you should always be in the middle of the brawls. You can be the initiator of the teamfights for your team, thanks to your Shadow Dash(E). Try to focus squishy targets, such as their adc and try to protect yours. Do not hesitate to use your ult on him in teamfights, the shield it gives is really decent and it can keep your adc alive for a few precious seconds.
If you play against assassins that can jump on your adc and kill him rather easily like Zed or Rengar, I suggest you save your taunt and also your ult to protect your adc. Since this kind of champion is particularly squishy, the 1.5 second taunt should be enough for your team to kill them before they can actually kill your adc.

I find this chapter really hard to write for a few reasons ; first, I have not played LOL for a long time, and there are still plenty of things with which I am not familiar with. Dueling is not an exact "science", there are many different factors that have to be reckoned with, and I think you learn to estimate properly each situation you are facing only as you gain experience (which I obviously lack).
In spite of my poor general knowledge of the game, here are a few tips that are useful in any 1 vs 1 situation :
-Always look at the enemy team's stuff, and compare it to yours. As a rule, you should avoid dueling someone who has more stuff than you.
-The same goes for XP.
-Obviously, you should not fight against anyone if you have 50% HP less than them.
-But if you catch someone mid-life, you should be able to fight him, even if he has more level and stuff than you.
-Avoid fighting someone who has buffs (blue, red or both) if you have none.
-Watch out for the indications that your team gives you about the enemy team (if they have their flash or ultimate for example).
-Always look at the map to see if other enemies are coming to help their allies.
-Be careful before going in someone with your E. This is your only escape so make sure you will not get trapped by the enemy team. If the enemy is really low, do not hesitate to use it to kill them, otherwise do not bother using it, unless you already know your target will not be able to escape.
-Indeed, Shen is not Master Yi or Jax : when I said that he is a good duelist, I do not mean that he can kill everybody 1 vs 1. By the way, you should rarely be able to kill the champion you are dueling unless he makes a big mistake. Shen is a good duellist insomuch as very few champions can kill him 1 vs 1, since all his abilities except his ultimate are good for 1 vs 1 situations. His passive gives him some damage, his Q sustain, his W a shield and his E halves the damage output of the target for 1.5 seconds. But he has too little damage and mobility to catch and kill those he duels (if they are full life).

Let us take a concrete example :
You play as Jax. You are lvl 18 and full stuff. You want to put a ward in the bush of the enemy blue buff but before you do so, you notice a (very stupid and depressed) Kha'Zix jumping on you. You just activate you E, your Z, you do tons of damage to him, he gets stunned, he jumps out, you jump on him with your Q and kill him. Then you put your ward in the bush. This is as simple as that.

You play as Shen. You are lvl 18 and full stuff. You want to put a ward in the bush of the enemy blue buff but before you do so, you notice a Kha'Zix jumping on you. Well, you do not really care, you go putting your ward in the bush and you just walk off. You are way too tanky for him to kill you, and if he follows you to fight you, thanks to your Q and W, he will lose HP faster than you will. So he will be forced to run away too. You will not be able to kill him, even if you E him he will just jump out after the taunt finishes, but he certainly cannot kill you nor force you to back off single-handedly.

Of course those situations are mere examples and do not give crucial details about the champions' stuff for example, or their cooldowns. This was only only meant to show what will probably happen if you face someone in 1 vs 1 with Shen.

Another very important thing, if not the most important, about dueling is the champion matchups. Like the rest, you mostly learn to know what champion you can fight in 1 vs 1 by playing a lot. This is precisely because I have not played much LOL that there are lots of champion matchups I am unable to discuss. I will try to add them on this guide as I gain experience and get to know them better. But if you want to know exactly what champions you can fight and what champions you should not fight with Shen, I suggest you look Shen's champion matchups on a guide posted by a more experienced player.

Shen is not hard to play mechanically speaking since he only has one skill shot in his kit, Shadow Dash(E) which is not difficult to land on enemies.
However there are a few special tricks that can be useful with this ability :
-The "Taunt-Flash" : This is the only difficult move to pull off with Shen ; it consists in using your Shadow Dash(E) and flashing during the dash animation. But why not simply Flash(F) and use Shadow Dash(E) afterward? If you do this, you will taunt the first ennemy or ennemies you encounter, but flashing while dashing can permit you to taunt ennemies behind the frontlane, so this is a powerful tip to initiate teamfights. However this requires a little practice before you can perform it properly.
Also, even if Shen is not the best champion to steal Drakes, since he can go through walls with his E you may consider E in (and even E-flash in why not?) to smite steal. If you land your Shadow Dash(E) on the ennemy jungler, he will be forced to attack you for 1.5 second, so he will not be able to use his smite. Keep in mind that this is risky though... If you do not manage to taunt the ennemy jungler, he will just smite the drake and you will be left alone in the middle of the ennemy team without your sole escape. Communicate with your team so that they be ready to follow you.
As a jungler, you should pay attention to what is going on all around the map at all time, and this is especially important with Shen because of your ultimate ability. However, you cannot be looking everywhere at the same time, so if you notice that one of your allies is low on health, you can just press R while pointing your cursor at his icon in the top left-hand corner of your screen.
Try to use your Feint(W) after Ki Strike(passive). Since your W lowers slightly the cooldown of your passive ability, you will be able to apply the bonus damage of Ki Strike(passive) more often.

There is not much to say about me... To be honest, I was quite reluctant to write this chapter as I do not like talking about me but everybody does it so...
Well, my name is roidescachuetes [kingofpeanuts in French^^ ....yes I am French :'(], I am unranked and I like to play for fun with my friends. I only play jungle and support. Since I only play for fun, I play champions I like (and I do not care if they are out of the meta), like Jax jungle, Master Yi jungle, Malzahar jungle (yes this is possible!! : look at this excellent guide if you are curious : http://www.mobafire.com/league-of-legends/build/master-1-ad-jungle-malzahar-the-voids-legion-394091), Twisted Fate jungle... and of course Shen jungle!

It is not that I think that many people will have read my guide thus far, but it seems normal to me to thank you anyway!^^
I hope that you enjoyed my guide and that you will try Shen jungle!

Even if you think this is lame, I am ready to listen to all your comments, as I really want to improve my guide for those who would like to read it.

Thanks again for reading! :)