Well, I took a look at all the builds made for Yi, and decided I'd add to the pile a little ^^
Basically, all you really want to do is add a lot of AS, which in turn (depending on what items you buy) will increase your MS substantially, for example Phantom Dancers.
Start off with Alpha Strike, (bear in mi9nd this is NOT a jungler build) in order to help farm and harass early game, remember to target creeps with it and not the enemy champ directly as you don't want to get too close to their tower, or them if they're decent players.
Next get meditate, you won't want to waste gold on health and mana pots, the build is rather expensive, but doable, and once you start pushing your lane a little, you'll need to run back to tower every so often to regain some health, as he is quite mana hungry, that's why you'll need clarity, forget exhaust, once you have boots you should be able to outrun any champ already, so no point really, hence the reason for clarity, heal is obviously for those tight situations where you've been slowed and your health is being chipped away faster than you'd like, of course you could just use meditate and have them do tiny incriments of damage while you regain more than they dish out, but that involves stopping entirely, so heal away.
I suggest getting wuju style after everything but your ulti is maxed, it's not an essential unless you're jungling, besides, the only time it is actually useful is when you can stay next to a champ and still attack while keeping up with them as they run in fear

Well, i'm not at the point of using runes, so i don't really have any suggestions for that, but i put up what i thought would suit him best, pretty much AS, MS and Critical damage.

Pros:
Extremely affective against ranged
Allows you to travel between battles quickly
High damage output
Maxed attack speed

Cons:
Very squishy
Hard against long ranged champs

Farming is relatively simple, wait for creeps to group together before using alpha strike, obviously, once you have life steal items, you're set on the health front, so use heal to support any other champs that need it