Whenever you play as Yi make sure to take mid no matter what! The only people that you should have a tough time in mid is against Mord, Teemo, or Vlad. When u take mid make sure you check what spells the opposing player has. If he has exhaust and ignite then its going to be really tough. Because Yi is a assassin/squishy type champ try to harass with him by doing alpha strike whenever their are only 3 minions and the enemy champion behind them because alpha hits 4 targets. When your yi the only way to go OP is to farm and kill even if u need to ks a little. Make sure you get last hits on minions for the gold and when alpha is maxed out it will deal decent damage and get u 4 minion kills in 1 second thats about 100 gold every time you cast that ability on minions. ALWAYS ALPHA AND RUN BACK BEHIND UR MINIONS UNTIL ENEMY CHAMP HAS BELOW HALF HP! Then you go in for the kill :P with alpha>wuju style and highlander if u got it. Always kills them unless they have exhaust or flash.

Once you get that kill or 2 its time to get the thing you lack most... MANA. Once lvl 7 you should be able to get the golem buff no problem. It is really really useful for yi. And you should have your boots and be finishing your Executioners Calling item. Once you have the buff keep farming that exp and gold im mid or go offensive and gank if you see enemy pushing top or bottom. Only gank if the enemy has below half hp cus they dont put up a fight and try to escape. When you alpha>wuju style> highlander> and ignite if their almost by their turrent.

We all know the massive damage yi can output on a enemy champion or tower. Keep getting golem and lizard buffs and go for dragon once in a while. You should have the item that KILLS! The infinity edge with it you take down turrents in seconds and get double kills in matter of seconds. Your Crits should be doing above 450 damage and your basic 160- 200 damage. With highlander's attack speed you should never die unless you get ganked. Make sure you keep farming until you have lvl 16 try to avoid team fights unless you know you have a chance. Let the teams fight in middle while you defend the sides by farming and back-dooring like crazy when everyone is mid. When you see more then 2 mia's recall its not worth dying with your buffs.
Once you killed about 200+ minions you are basically op and should start jumping those enemy squishys! NEVER EVER GO AGAINST TANKS LIKE MORT SHEN OR RAMMUS ALONE! You will die unless they are terrible and dont have thornmail. The worst thing to happen to you is to get stunned! Enet team battles when you see your team is lossing or both teams have about half hp so you can get that triple or QUAD kill you really want!

Once in late game your stats should be impressive unless they have Really good tanks. They will try to kill you first after getting legendary make sure you go in kill one or two and use ghost to either escape or to get a triple. With starks it will be hard to kill you and if you cant buy anything else substitute Executioners with Wormong's and Berserker greaves with another phantom and take out their inhibitors and teamates before they even BLINK!