In masteries I take 21/9/0. Master Yi benefits from 21 points in offense so much, that going any other way is just absurd. You get all that extra damage and survivability, you can't deny that. I use 21 in Offense both for laning and jungling.

Yo Yo Yi is all about Attack Speed. Master Yi is a very fragile, potent strike fighter, so this build is about:

catching players off-guard

dumping every cooldown

after they have used their cooldowns

avoiding certain debuffs

ducking and weaving through the map

and surgical strikes on unguarded turrets.

The Item build is the most important part of Yo Yo Yi. If you observe big pro-lol teams like Dignitas, they love their daggers. 15% Attack Speed is incredibly cheap for the price. So after you get your

Lantern

and Boots,

buy 2 or 3 daggers. This will tip you over the edge into awesome.

Start kicking butt, working your way to a Frozen Mallet.

Then start upgrading those daggers one at a time into a zeal -> phantom dancer.

zeal -> trinity.

zeal -> phantom dancer.

To give yourself some needed range early on, take Alpha Strike for your first level. Wait for enemies and minions to get far enough away from the turret and launch alpha strike. They have a quick cooldown, so just be patient and 'duck and weave' until you can use alpha strike again.

The early game is about avoiding death. If you are lucky you will take damage but not enough to kill you. Back up to a turret and recall. Heal up then come back for more. You will be a small threat early game, so be patient and don't get turret sniped by chasing enemies to far in. You are very squishy, so just be careful.

Since this build eschew's mediate, you do not have a quick heal mechanism, so running, recall, and healing at your base is what is important.

After the initial Alpha Strike is taken, max wuju style when ever offered, but Highlander takes priority over all other abilities and should be taken immediately.

Ignites target enemy unit, dealing 70-410 true damage (depending on champion level) over 5 seconds and reduces healing effects on the target for the duration.

This is another great spell for Master Yi. It deals a lot of damage and is great to finish off champions with this extra damage. Ignite is also useful because of 50% Healing reduction. With Ignite you can shut down enemies like Swain(**** on their healing capabilities) easily.



Exhausth

Exhausts target champion, reducing their Movement Speed by 40%, attack damage dealt by 70%, and ability and item damage dealt by 35% for 2.5 seconds.

This is great spell and like Flash and Ignite, works miracles both in offensive and defensive tactics. When chasing, this spell reduces enemies Movement Speed by 40% for 2.5 seconds, which makes it a great offensive tool. When using it defensively, this tool protects you by nerfing attacker's basic attack damage by 70% and item/ability damage by 35% for 2.5 seconds.



Cleanse

Removes all disables and summoner spell debuffs affecting your champion and lowers the duration of incoming disables by 65% for 3 seconds.

While I rarely use it, it can be still viable on Master Yi. This champion is extremely vulnerable to CC disables and if you do not intend on buying Quicksilver Sash, this spell is kind of must take. Since November 2011 it also removes Ignite, which is great.



Smite

Deals 445-870 true damage (depending on champion level) to target enemy minion or pet.

This is obviously a must pick for junglers. Not only it helps you jungle faster than you could with any other spell, it also grants you amazing buff controls. I can't stress how important smite is, I personally have stolen Baron more than 10 times with Smite.



Flash

Teleports your champion toward your cursor's location.

One of best spells IMO. Its useful both in offensive and defensive tactics. You have been hopelessly ganked? Flash away. Enemies chasing you? Flash over the wall. Enemies got away with 50Hp? Flash to them and deal 1 more attack. I love this spell, even after the nerf in November 2011.