Hello Summoners!
I'm Theomathel, and I've been playing League since mid-2013. I've greatly enjoyed the game, and hope to share some of that with you. When viewing this, I recommend you pull up the cheat-sheet in the top-right of the guide to quickly refer back to my build. Also, this guide is relatively dependent upon not jungling against another Shaco. If this occurs, you should follow a normal jungle Shaco build. Without further adieu, here's my counter-jungling Shaco build!

Pros:
-Very powerful early game
-Strong gank potential
-If done correctly, you control two jungles!
-Quick camp clear with Jack In The Box
-Good excape with Deceive
-High skill cap

Cons:
-Weak late game
-Requires early kills
-Requires lots of vision and communication with team
-High skill level

Recommended:
Flash: Standard first spell-pick for all positions. Great because of its escape and chase utility.
Smite: You probably shouldn't be reading a counter-jungling guide if you're not going to take Smite. All the items are meant to get you into a position to smite creeps in the enemy jungle. You make yourself much more vulnerable by not using this.

Optional Replacements for Flash:
Ghost: Still gives you escape/chase potential with a 90 second shorter cooldown.
Ignite: Can be used to set up early kills in enemy jungle.
Clairvoyance: Good vision spell to figure out where the enemy jungler is.

Please don't take Clarity on Shaco. He has low mana costs to begin with, let alone having a blue buff.

Backstab: Pretty self-explanatory. This has great utility when chasing enemies.

Deceive: This is one of my favorite abilities in League. It combines two escape tactics, blinking and stealth (not to mention the guaranteed crit), into one short-cooldown ability. Use this to engage initially, then keep it off cooldown to escape in case things go awry.

Jack In The Box: Probably the ability Shaco is known for. These boxes do decent burst damage, and can help you clear camps quickly. I'll describe their usefulness for counter-jungling later in the guide.

Two-Shiv Poison: Another great ability. Just one rank in it gives monsters a 20% chance to miss! The slow also works well with Shaco's passive, allowing you to get more high-damage auto attacks in (usually even a kill). The active isn't the best on this skill, however, and should not be used on monsters when clearing camps. The damage isn't worth losing the passive chance to miss.

Hallucinate: A walking time-bomb that also attacks! Unlike Summon: Tibbers, you can send him into a group of enemies and deal a huge amount of damage when they die. Also good defensively for buying time as your team retreats.

I'll admit it, I love utility masteries. You can, however, use whatever masteries allow you to function best in the game. What I've shown is what I use, but you can use whatever you wish.
Just like masteries, you can use whichever runes you choose. I definitely recommend movement speed quints though, because they allow you to get in and out of the enemy jungle quickly.

Masked Shaco is required for this build to work...
Just kidding.

For Shaco jungle specifically, there are three phases of the game. The first is the bully phase. Second is the gank phase. Finally, you engage in the team-fight phase. Each of these are described in more detail below.

The bully phase is where Shaco gains a huge early-game lead. This is the most important phase, but usually only lasts 3 or 4 minutes. Here's how I usually play it out:
0) Try to figure out if the enemy jungler is starting near red or blue (could be doing Gromp/Krugs).
1) Start at blue buff. Begin placing your boxes down 60 seconds before the buff spawns, since the boxes last 60 seconds when not attacking. Try to get a quick leash if you can. The boxes allow you to quickly get the buff and level 2, while the enemy jungler is still doing their first camp. They should be pretty low on health at this point, so you have to act quickly.
2) Gain vision of the enemy, using a box, ward, or Clairvoyance. Use Deceive to gain invisibility so they don't see you coming.
3) Kill them.

Note: This may not work every time, but as long as you have vision, you have a pretty good shot.

This is the standard ganking phase for a jungler. Use Shaco's Deceive to surprise enemies, and burst them down. Shaco is very strong during this phase, due to his surprise. Try to let your laners get the kills, since Shaco falls off in the late game. Focus more on ganking than on farming, since most of your ganks will be successful.

The team-fight phase is, as previously mentioned, where Shaco falls off. Here, you can either switch to a tankier build, or focus on picking lone targets off with incredible burst. Shaco will not work as a front-line fighter however. If you played the gank phase correctly, your laners should be more than powerful enough to make up for your power fall.