Master Yi is considered by many to not be a viable champion outside of low Elo. I beg to differ! Master Yi can control the entire flow of a game and single handedly take the enemy team apart by him self.
Many people will build straight damage on Yi then charge straight into a team and instantly die then they yell THIS CHAMP SUCKS! Please don't do this.
Yi is thought to be a simple champion and most people fail with him due to this thought process. Yi has many tools to win the game, just make sure you use them properly.
Here are some Yi replays at High elo
The link is a nice little video stonewall008 made giving a synopsis of Yi
I use standard dps jungler runes.
Armor pen reds and quints, armor yellows and magic resist per a level blues. You can throw attack speed quints as you please, it will help you jungle faster but hurt your mid/late game more.
I use standard 21/0/9 for jungling. Nothing too special with my masteries here.
For my starter items I get cloth armor+5 potions. After completely my first route through jungle I grab boots and 1 or 2 more potions.
Grab wriggles Avarice and then your merc threads. You can go zerkers if the enemy team has little to no cc but I don't recommend it.
If I am farming really well I will straight rush my youmoos, and if I am not farming well I will grab phage before completing youmoos.
Once youmoos is done I finish trinity, after trinity you can either build more dps or start getting tanky. It really depends on what your team needs.
Now I know you will look at my item list and the first thing you will say is trinity! WTF?
I used to hate on it too but it honestly gives Yi everything he needs.
Master Yi Skills
I max Q and E first for faster jungling/killing and I save w for wayyyyyy later in the game.
Q-Alpha Strike has many uses beyond just farming minions and closing gaps on enemies. Alpha strike can be used to semi dodge cc as well as big incoming damage. Take for instance Lets say sion is running at you and you know he will use his stun combo on you. Be ready for him to stun and when he does alpha strike to something while the projectile is in mid air. You will be stunned while in alpha strike animation so when you come out of alpha strike the stun will be over. This will also cause sions shield explosion damage to be missed usually as well.
This is just one example of what you can do with alpha strike. Go out and experiment!
E-Wuju Style is Master yis built in steroid move. Make sure you press E the second you start attacking someone to maximize its effect.
W-Meditate is very useful in team fights late game but it is practically useless early.
W can be used in a team fight once all the cc has been thrown out if you are getting focused you can go ahead and W. MAKE SURE YOU PAY ATTENTION TO WHO HAS WHAT CC UP. If you can sit in W the entire time you can tank almost an entire team beating on you. It is a beautiful bait if done right and will win your team many fights.
R-Highlander this move is amazing. Not only does it put yi on steroids making him run at light speed and attack insanely fast but it also makes yi immune to all slows. This move can be used for chasing people down as well as getting away.
Highlander also makes Yi a great candidate for back dooring :D
I like flash/smite on Yi but I have been experimenting with teleport/smite for super split pushing. Both summoners work well so it is all preference.
Creeping / Jungling
This video is made by stonewall008 so the credit goes to him
I follow the first route in the video 9 times out of 10. There are also other paths shown int he video as well.
You can also start at enemy wraiths or your own blue with a pull. I believe the first path is the best path however.
Yi mid/late game
Yi is pretty poor in team fights unless extremely farmed. Outside of cleaning up a team fight you should usually not participate in team fights heavily.
Instead you should be split pushing lanes constantly farming. Your goal is to farm EVERYTHING on the map and have more farm than anyone. Keep constant pressure on lanes at all times. When you are split pushing also use wards to keep yourself safe and see the enemy coming.
If you are pushing bottom lane then ward the brush near the lizard camp. If you are pushing top lane then ward the bush at blue golem. Reverse this if you are playing blue side.
Try not to engage people while you are split pushing unless you know where everyone else is on the map. Yi can kill almost anyone 1v1 however don't be greedy and get caught chasing down a kill. Your main goal is to keep farming and to push towers down not to grab that kill. So keep that in mind!
Once you have your core items and are farmed a god awful amount you can participate in team fights but until then you choose the fights because you have your ultimate and can't be caught. So let the enemy play your game and bait them around the map.
One of my favorite tricks with yi is to tell my team to wait somewhat near baron. Then I push bottom lane really deep and bait the enemy team into chasing me. I laugh and taunt the enemy team as I run away while my team gets a free baron. You would be surprised how often this works.