OK, this is my first build, inspired by my friend Castpast. Essentially this build takes full advantage of Alpha Strike and Meditate, making it incredibly hard to kill Yi while he decimates the opposing team.
Early game is very farm dependent, so don't try to get first blood unless you know your team will be able to back you up. I start with an Amplifying Tome and either a health potion or mana potion, depending on the make up of the other team. Staying in lane as long as possible is crucial since you are still quite squishy in your early levels. Getting the Hexteck Revolver is really nice for Yi, since not only does it provide AP it also give Alpha Strike spell vamp, making it possible to stay in the lane longer. When you get to around a quarter/third life run away and use Meditate. This ability, especially in later levels, is so OP when you go this route you can go from less then 10% Health to full just from channeling. In this since you are very tankish. I've taken the brunt of Warwick's ulti and stood in Morgana's pool and still gained health.
Once you get to the middle levels you should probably grab the Golem Buff whenever available. This allows you to spam Alpha Strike and makes it nearly impossible for you to die.
The masteries are a bit spread out, but for good reason. The magic penetration and AP boost in offense is needed to make farming creeps more effective, while the dodge/defense makes up for the lack of armor. Having improved Cleanse and Ghost means that you should never be DPS'd down and gives you a second escape/chase tactic.
Note: I usually don't get Wuju Style until after 6, but if you find yourself encountering silencers you should probably drop a point into it at 4 and pick up the missed Meditate point at 7. Zhonya's Ring also helps against casters, which is Yi's biggest weakness.