This is my build for Yi. I Rarely lose with him, and this build serves me very well. allow me to explain



Skills - pretty self explanatory. first get wuju style then switch it off with alpha strike, getting highlander whenever you can. I never use meditate because the times i tried i died mid fight, but if you are low on hp, your teammates are on their way, and you can't get away, might as well use it. bonus armor and hp regen are nice. Wuju style is very nice because in the early game it allows you to have a higher attack while being able to buy non attack items. Alpha strike can be used to kill minions quickly, or you can use it on a minion right in front of an enemy champion, hitting them then running away just as fast.

Runes- I went for pure attack speed and critical chance. if you can hit them fast and get critical out, they will be dead before you know it. You can also maybe go with damage or health, depending on your style of play, but with the health runs i really doubt 40-100 extra health will make much a difference late or early game :/

Items - i always always buy berserker boots first. Yi needs the mobility being a pusher and a good champion killer. Starks will give you another 40% attack speed along with health regen and life steal, which will keep you in the lanes much much longer. Yi doesn't have a whole lot of survivability, so aegis is a nice, cheap item to boost your defense and hp. it has saved me sooooo many times. and finally infinity for the attack power and critical. This is where all those critical runes and masteries come into play. your criticals now do 250% and this gives you an extra 20% chance. You will be hitting pretty high. After that, you get what necessity dictates. If there are a lot of casters, banshee would be a good idea. Get guardian if you can't seem to stay alive, or if you are feeling really bold, try for a bloodthirster. I normally end the game before i can even complete infinity.

Summoner Skills - these two skills are very handy, yes you may be mad because heal is there, but just read further. Heal can save you in a fight, any part of the game. Scenario - you are fighting 1v1 with another physical damage dealer and you are at 1/4 life and they have 3/4. if you heal, you can go up to 1/2, then you can use exhaust to blind them for three seconds. Hitting with those criticals and having the high attack speed, you just turned the battle in your favor. Also, heal can save an ally, because if he is running and you step in, use heal, give him some health and try and halt the attacker. Works for me :D

play style - Yi can work either solo lane or with an ally. Try and farm as much as possible by getting the last hits with minions. If you are laning against a harasser, it would be a good idea to put 1 point into meditate so you can get 100 or so health back, and to also buy health potions. Yi becomes a lot more effective once he has beserkers on him, but starks is what makes it work a lot better. save up for aegis, and depending on who you are fighting, be it a mage or physical damage dealer, get whatever you need for aegis first. save up for the bf sword, and by this time you should have around 210 attack damage due to Wuju Style. Get in a lane with all your allies and try and do a major push, or you can play defensively and kill off a few of their guys then push, and hey you may even score an ace, have an awesome push, and call it good game. But always remember, you can always turn the tide of battle, defensive play can score you gold and experience, allowing you to catch up in items and levels, and also scoring an ace in you base will allow for a major push late game, due to the high respawn time. With Yi, don't get into a 2v1 or 3v1, he just doesn't have the survivability for that kind of fighting, but in a 2v3, you can and your ally can kill 1, even the playing field, then take the rest out. Yi is a high damage attacker and he can hit very very fast, but be very weary of disables. If you see yourself at 1/2 life, by yourself with 2 or 3 enemies on your tail, use highlander to give you the extra speed boost to get out of their alive. The cool down on it isn't very high, so you can use it again very soon. Never forget that highlander has the instant cool down if you kill an enemy with it.
P.S. watch your mana, if you are at about 200-300, go back to base and let it replenish, because a Yi without mana is a Yi without options.

I'm open to comments, ideas, and the like. Just give me a shout as to what you would do and what you think needs improvement. I use this build all the time and I have a 80-90% win rate with Yi. Play smart and you can win.

*update* Thanks to comments phantom dancer seems to be the way to go, with increased attack speed, movement speed, and crit chance, it is much better. I tried it, and yeah it works nicely.

*update* Added a more in depth build sequence, going to soon ad a better description to play style.

it is very good for your yi to have a good attack, and to have a high critical chance. Some may shun me for this but it works :D Cool down reduction is very handy especially for those last hits with alpha strike. cripple is a major helper, seeing as though it makes exhaust more potent and it can really turn the tide of a battle in your favor, or just stop a guy in his tracks. Menders is a must for you to be able to use heal like you need to, it is a very effective spell and even more effective once you give it an extra 30 second cool down.

pretty self explanatory. first get wuju style then switch it off with alpha strike, getting highlander whenever you can. I never use meditate because the times i tried i died mid fight, but if you are low on hp, your teammates are on their way, and you can't get away, might as well use it. bonus armor and hp regen are nice. Wuju style is very nice because in the early game it allows you to have a higher attack while being able to buy non attack items. Alpha strike can be used to kill minions quickly, or you can use it on a minion right in front of an enemy champion, hitting them then running away just as fast.

these two skills are very handy, yes you may be mad because heal is there, but just read further. Heal can save you in a fight, any part of the game. Scenario - you are fighting 1v1 with another physical damage dealer and you are at 1/4 life and they have 3/4. if you heal, you can go up to 1/2, then you can use exhaust to reduce their damage for three seconds. Hitting with those criticals and having the high attack speed, you just turned the battle in your favor. Also, heal can save an ally, because if he is running and you step in, use heal, give him some health and try and halt the attacker. Works for me :D

Yi can work either solo lane or with an ally. Try and farm as much as possible by getting the last hits with minions. If you are laning against a harasser, it would be a good idea to put 1 point into meditate so you can get 100 or so health back, and to also buy health potions. Yi becomes a lot more effective once he has beserkers on him, but starks is what makes it work a lot better. save up for aegis, and depending on who you are fighting, be it a mage or physical damage dealer, get whatever you need for aegis first. save up for the bf sword, and by this time you should have around 210 attack damage due to Wuju Style. Get in a lane with all your allies and try and do a major push, or you can play defensively and kill off a few of their guys then push, and hey you may even score an ace, have an awesome push, and call it good game. But always remember, you can always turn the tide of battle, defensive play can score you gold and experience, allowing you to catch up in items and levels, and also scoring an ace in you base will allow for a major push late game, due to the high respawn time. With Yi, don't get into a 2v1 or 3v1, he just doesn't have the survivability for that kind of fighting, but in a 2v3, you can and your ally can kill 1, even the playing field, then take the rest out. Yi is a high damage attacker and he can hit very very fast, but be very weary of disables. If you see yourself at 1/2 life, by yourself with 2 or 3 enemies on your tail, use highlander to give you the extra speed boost to get out of their alive. The cool down on it isn't very high, so you can use it again very soon. Never forget that highlander has the instant cool down if you kill an enemy with it.
P.S. watch your mana, if you are at about 200-300, go back to base and let it replenish, because a Yi without mana is a Yi without options.