Opening Comments

Alright so this is my first build and I discovered this while I was screwing around with Ashe in All Random All Mid, and I tested it out in a real game. It works pretty well. Depending on whether you want to go with life steal or critical damage, building infinity edge and bloodthirster are interchangeable. If done well, you should be able to easily get 10-15 kills in a game with her.
Ashe was my first champion I bought and once I got a higher summoner level, she didn't work out for me so I stopped using her. But now this build has worked great ever since I started using it.
I used a similar build with Twisted Fate, and it also worked, except that I bought a Bloodthirster instead of an Infinity Edge.

Introduction
This build focuses on critical damage and attack speed for Ashe. The mixture of critical damage and attack speed blends very well together which allows you to kill enemies fast and efficiently. The critical damage specifically helps when killing tanks with high armor levels. You should regularly do 500 dmg once you get your infinity edge and bloodthirster. Combined with the phantom dancer and life steal, that is a deadly combo.

Masteries

Alright, Masteries should mostly go offensive to increase Ashe's effectiveness. Feel free to experiment with the masteries because I haven't really looked into them that much.

Runes

I haven't done any experimenting with runes, but all I go with is attack speed, but I think attack damage would also work well with this build. I would recommend attack speed though. It gives you an edge over other champions in the beginning of the game. I think a combination of attack speed and attack damage would work well together. Critical runes would also work well to give more of a crit. hit chance in the beginning of the game. Getting Mana runes would also be a viable choice because of Ashe's low mana. Its hard to spam her Volley in team fights when she has such low mana.

Summoner Spells

You can go with any summoner spells, but I like to go with teleport and flash. Teleport allows you to get across the map quickly. It also allows you to combo with your Crystal Arrow from across map and kill champions.
Flash is a good item for dodging and fleeing from enemy champions when you're low on hp. Ghost would be a good spell for this too, but I like flash because you can go over walls.

Item Build

Since this build is based off attack speed, I start off with a dagger and a health potion.
A health potion will keep you alive, and a dagger will increase your attack speed. By the time you have to go back, you should have enough money to turn that dagger into a zeal.

After getting a zeal, I rush to get berserker's greaves. After getting boots, I buy a pick-axe. I get the pick-axe first for the infinity edge because its relatively cheap and the damage can sustain you until you have enough for the B.F. sword.

The Infinity Edge is the centerpiece of this build. The 250% critical hit damage really makes this build along with the 20% critical chance. After the patch, I feel that this has become a much more affordable item and is very good.

After completing the Infinity Edge, you should complete the Phantom Dancer. This provides you with a bigger chance to critical hit someone. If you end up getting a bloodthirster and you want to stay alive longer, I recommend starting with the vampiric sceptor so you can get some life steal. If you're not worried about being so squishy, just go with the B.F. sword first to do 500 damage critically.

As for the frozen mallet, I suppose if you really need the health, it would be better to get that instead of the bloodthirster. Although I never actually played a game long enough to get it for my sixth item. You could also go with a different defensive item depending on if their team is mainly magic damage or physical damage. A Banshee's Veil or Force of Nature is good against magic damage while a Thornmail is good against physical damage. Banshee's Veil blocks one negative spell every so often and a thornmail returns 30% damage taken. A Guardian Angel provides a combination of both magic resistance and armor. It also revives you when you die for 700 hp.

Tips

Try not to initiate in combat because you are very squishy.
Your volley should be your main ability which will help in getting kills, along with frost shot.
Frost shot helps slow the enemies which will allow you to cover ground on them and use your volley. This also helps to slow them when they run away when they are low hp. Remember that frost shot also works when you use volley, so it will help when team fighting.
It would be better to lane against more squishy people. I think this build works for solo lanes better, because of the ability to farm and get more money.
If you die too quickly and want stay alive longer, get a frozen mallet instead of a bloodthirster.

Counter

A good way to counter this build would be to stack armor. A lot of armor. Also, when its good to gank Ashe in the beginning so she falls behind in leveling and building. If she gets fed in the beginning, it is very difficult to stop her.
You want to starve her from getting creeps in the beginning, mainly to keep her from getting gold. Getting her behind in her build will help greatly.

The End
That's the end of my guide. Feel free to leave comments and feedback, but try not to be a **** about it because, remember, this is my first guide.