Flash: Must have. Provides safety, repositioning and chasing. NEVER EVER EVER take something else.
Heal: Standard for ADC. You really need that safety and it gives you a really great movement speed buff for you and your support. Try to use this before flashing when you are getting ganked.

Statikk Shiv: for farming, poking and extra damage.
Bloodthirster: for atack damage and ofcoure its amazing life steal and regen.
Infinity Edge: couse Ashe needs atack damage and critical hits to win.
Phantom Dancer: mostly for the atack speed and the chance to get critical hits. also the movement speed and the UNIQUE Passive are also realy welcome.
Last Whisper: for the armor penetration, couse if an enemy see what you are buildingit will probably get some armor.
Berserker's Greaves with Alacrity enchant: as I said in some circumstances Ashe needs to flee or maybe go for the kill (ONLY IN LATE GAME), then this enchantment is realy usefull. the boots I chose for its atack speed.

Runaan's Hurricane is fast yes, but as fast is so useless it is.
seriously DONT take it.

What I recommend you to do is go for your Volley every time you can, except if you can go for your Enchanted Crystal Arrow.
When W is not available go for your Frost Shot. Your Hawkshot is the least importand of all but always usefull to make sure the area is clear.

This is the standard ADC masteries that I use. You should always take 21/9/0 because 21 in offense is really needed for the extra offense (you dont say) and the defense is needed more than utility for surviving poke, teamfights and everything that involves the enemy team basically.

Basic ADC runes.

Earlygame: PLAY SAFE!!! Ashe is realy weak in early game, if you play agro, your dead. The enemy get feeded, your team gets angry and 100%losing guaranteed.
Just farm ALOT, poke an enemy champ from time to time and make sure you follow the item part of this guide.

Midgame: Same thing, keep building, engage sometimes (MAKE SURE YOUR NOT ALONE).
AKA keep getting more powerfull.

Lategame: NOW YOUR FULL BUILD YOUR THE NIGHTMARE OF THE ENEMY!!
Go ahead engage the hell out of the enemy, push every lane, fight the dragon and fight Baron.
WIN THE GAME!!!

Teamfights: Keep distance. You are an Archer so fighting a melee champ is easy if you hit them on distance.
Try to be with your support the whole time in early and mid game.

Ashe can also be a support, and i made this guide for it: http://www.mobafire.com/league-of-legends/build/team-ons-ashe-support-414848

:: Soraka::

Explanation: This is a very passive lane combination as Ashe gains a lot of bonus gold while farming. Soraka augments the ease of farming by being able to heal and buff Ashe with Astral Blessing and Infuse.

They have very little damage compared to other combinations and aren't likely to even try to fight as they are stronger late game than early game.

Neither has a reliable escape except Flash, with the possible exception of the heals and buffs from Astral Blessing.

Starting Doran's Blade also aids in last hitting, which is your primary goal, but may leave you susceptible to ganks. Lane isn't difficult in that you just have to farm, but if you get engaged onto it may be hard.

Damage: 3/10
Safety: 7/10
Difficulty: 6/10