when playing ap yi, the main thing you need to do is know when to jump in on team fights, you always wait for a champion to get to low health before you jump in.

when Yi's ult is up and you kill someone, every cooldown on your skills reset.
AND!! yi's (Q) jumps to 4 people so you can be miles away from someone you want to kill but if there is a line of people you can use it and get to that person.

staying in mid is essential, never try to gank someone untill lvl 6. once you git that and you see someone with semi low health you can go top or bot and easly get a double kill, with the (R)(Q) combo.