Ashe, in my opinion is one of the best midders in all of League of Legends, the others being Caitlyn, Karthus, Vladimir, Malzahar, Miss Fortune, Rumble, and Lux on a good day. This build has proven to be quite successful for me and i rarely lose in the mid with it. It focuses Damage above all else, with DPS and critical strike thrown in and a hint of lifesteal. While many people focus her more DPS, i have found that her slowing capabilities require less need for attack speed. Her range, with frost shot and a ton of damage will bring down any enemy who tries to run.

Masteries

21-1-8 is the way i set my masteries. This maxes out the offense tree, boosts the heal spell in defense and gives 8 points of mana, health, and death-time reduction.

Summoner Spells

I always bring heal and clarity when midding with Ashe. 2 "noobish" spells i know, but these help early game survivability a ton and will keep you in your lane for a long time. Ashes mana pool is small and runs out quickly early on. clarity helps you keep ripping off those ECAs and harassing with plenty of volleys.

Runes

For my runes i simply use armor penetration marks and quints, cooldown reduction glyphs, and mana regen seals.

Build

Start with Doran's blade for a health and damage boost that will help you stay aggressive early game. wait, if you can, until you hit 1000 gold to go back for boots of swiftness. If you are doing exceptionally well, wait until 1650 to get a B.F. sword, but if you must go back earlier and have over 975 gold at least, pick up a pickaxe. these two items will give you 70 attack damage. After this finish Infinity Edge as soon as possible, then focus on Phantom Dancer. Zeal is a great item to pick up first then finish the rest of PD. Next, you can either go for a Bloodthirster or a Black Cleaver. If you feel that enemies are getting away from you to easily, get a B.C. for the attack speed. if you need more punch, get a Bloodthirster and farm to get your stacks up and really boost your AD. eventually you should have both items. Finally finish with some crit and damage from a Youmuu's Ghostblade. Stacking with Sword of the occult is optional... i only reccommend doing it if you are having an extremely good game. at 20 stacks you will have an extra 100 damage plus a movement sped boost. If you finish this build, you will have maxed out your crit chance, have a damage over 400, attack speed of around 1.8 and have a decent amount of lifesteal. Squishy, but with frost shot, high AD and her movement speed you will be set.