Master Yi is considered by many to be the easiest champ to play and, in the hands of the inexperiencecd, easiest to kill. I will not deny the later part because of Yi's extreme squishiness but he is not as easy a champion to play as many believe. Yi's speed and late game AD make him one of the more difficult champions to master because of his versitility. With this build I have found that you can build Yi to withstand most AP and AD champ attacks and survive long enough early game to farm it up a storm. Not saying this is the perfect build for Yi, but I've found in most matches when I build this way I can kill quicly, survive longer, and eventually be the carry the team needs. Towers? What Towers? lol

Pros:
AD God
Excellent pusher
Late Game beast
Wuju Master

Cons:
Squishy as all hell
Not the best team fighter
Hard to time certain abilities

So as many of you have no doubt noticed, this build's mastery tree is heavily focused in the offensive tree. I do this because Yi is an offensive champ -_-; he can jungle like a boss but I'm not that good of a jungler so making a guide that even mentions jungling as an option is out of the question. The masteries are basically there to beef up you damage at the start and boost you at the end so your speed and power are unparalleled.

Flat runes I've found are better for Yi because of an early need to establish dominance over your opponent. When Yi is fed, the other team usually quits by 20 mins due to his sheer overwhelming presence on the battlefield. When feeding as Yi you still have the opportunity to deal assist damage because of armor pen and AS and maybe even get some KS' if you really must get yourself back up to speed with your lane mate.

At the beginning I was all for the exhaust and ghost spells. they enabled me to kill and avoid dying pretty much every match. For a while i tried exhaust and rally for early game beastification but that didn't work out at all. Then a friend suggested I use the exhaust/flash combo to help with jukeing enemy combatants and at first i was skeptical. Flash had a nice range but I play with the camera following me champion so its hard to switch between locked and unlocked cameras while running away. Then i remembered flash works through the walls too. This changed up how I played entirely. When an escape is truly necessary or an enemy is about to get away, flashing gives the extra boost necessary without wasting an ulti. And with exhaust as a way to slow them down further I began getting kills like there was no tomorrow. Again this works for me and some may still prefer ghost. I'm not knocking either choice because both are good but for ether you'll need exhaust.

Since Yi is an AD carry you gotta make sure your AS is covered. I've had games where because my AS was just a bit better than another Yi or especially Warwick, I've been able to come out on top. Start every game with the dagger and a potion. When laneing with a ranged champ like ashe or morg that AS early game usually means a first kill for Yi and early game gold! We all love that extra 400 in the pocket don't we? From there get your berseker greaves and, depending on your farming status, a couple more health pots to help you stay in lane longer.
Now at this point you're probably thinking, "Here comes the Phantom Dancer!" and that's where you'd be oh so wrong. Phantom dancer is an excellent second item to build for, don't get me wrong, but I recently discovered that Wit's End was a cheaper and faster way to get some much needed attack speed while building towards the ever lacking magic resistence. 40% AS with a 30+ magic resist bonus and a passive that grants 6 MR per hit up to 4 stacks as well as 42 bonus magic damage per swing? That sound like a good replacement to me.
At this point your gonna be attacking the enemy at crack head speeds so it's time for some AD. The B.F. Sword is the ultimate body destoryer for str8 up AD. Seeing as how the enemy team will be a bit confused as to why yi is building such an item set this is when you really throw them off and build that Zeal. The crit chance, AS, and movement bonuses will be a great help when getting mid to late game and team fights start becoming a norm. NEVER INITIATE AS YI UNLESS YOU ARE COMPLETELY SURE OF YOUR VICTORY! I can't tell you how many times I've started out like a beast then, due to my over confidence, I try to solo gank a team only to get shut down like a noob. Only fool's rush in I know, but wen your at 6-0-0 and it's only 8 mins in you feel like a god.
After zeal is obtained go for Infinity edge. As probably the best AD item in the game it's a must for any Yi. Phantom dancer is next because by then you should have a good number of kills and assists under your belt and with a bit of late jungling a good amount of gold in your pocket. From here your wanna build defensively because you're already such a AD juggernaut.
Emblem of valor is cheap and gives 17% life steal; With the amount of damage your dealing, and the frequency at which you'll be criting your opponents your heath should cease to deplete in 1v1. After you've got the emblem go for the Fervor. You'll help out your teammates with the 30 per 5 hp passive and the extra armor pen it delivers as well as getting that necessary extra life steal.
Now if by some random act of lunacy the game isn't over by this time, go for thornmail. It seems like a troll thing to do but when a match is edging on 60+ mins every advantage counts. The 100+ armor and 30% magic damage rebound make it almost impossible for Yi to be stopped, by tower or by enemey champ.
Red and blue bluffs are nice additions to this rotation when your in a pinch or want to help carry your team over the edge to victory. And I feel that by covering your magic resist and armor absences you'll survive a lot longer and help your team more than by just going straight for the AD/AS combo.

Building in this sequence means maxing damage while maintaining lane dominance. Mediate is best at lvl 5 where it grants 300 magic resist and armor (meaning late game with wit's end and thornmail that yi is basically unaffected by damage of any sort) so save it for last; speed lvl it. Alpha Strike is first because control of minions means early control of a fights influences. Wuju style passively ups your AD and the doubles it when used. As an AD champ this is almost an unfair advantage -_- don't change it Riot! Build your wuju up first and maintain your edge in the AD pool. Highlander is probably why so many people think Yi is easy to use and is a noob champ because it ups your attack speed and with a kill your cooldowns are refreshed meaning, with proper mana control, destruction in Yi's wake. The key to Yi is to know when to use your abilities and thankfully I've figured out a way to do so that won't destroy his mana pool:

When 1v1 ing:
Highlander >> Wuju >>>>>> Alpha Strike (only to chase or as a final edge for the kill)

When 2v1 ing:
Highlander >> Wuju >> Meditate (if high enough lvl; means life or death at the hands of a good team) >>>>>>>> Alpha Strike (again best for chasing and final blowing)

Other than hat Yi's basic attacks should be enough to help overcome most enemies

In closing I;d like to say that Yi is a great champion that lot's of people don't give much credit to because his skills seem so cheap and easy to use. They aren't all that easy to master but when used properly Yi becomes IMO one of the best champions in the game. I'm no pro at this and I don't often play ranked matches but this is a good build for those who want to master the Master of Wuju. Next time you're in a match just remember "Yi's Blade Is Yours"