Ashe is in my opinion an AD carry that anyone must know how to play. She is ridiculously cheap for her potential, specially because she is easy to learn but not that easy to master. In this guide i'll try to explain the difference between knowing how to play Ashe and mastering her.

Pros

Epic farmer
Crit on early level
Stun and slow
Scout ability (prevent ganks and Baron and Dragon check)

Cons

Squishy
Slow early game
Gets targetted (like every carry)
No escape ability more than Volley

Why chose this setup of Runes? I like this full AD build because takes lots of advantage early game. If you start with Doran's Blade and this setup of Runes and Masteries, you'll start with +33 AD, great advantage for those who crit (Gangplank, Ashe, Tryndamere).

Besides, in my opinion, Runes take a decisive stance in early game, thats their objective. Maybe +33 AD is not a great difference when you have 300 AD (or 333 in this case), but means a lot when you have a starting AD of 50-55 and you can take that to 80-85.

Why go 25-5-0? First i'll explain to you why dont get what i dont get :D. Except for Summoners Wrath and the AP based Masteries in the Offense Tree, all of them could go with an AD champ. I'd rather avoid the Demolitionist mastery cause of the cost-effect ratio. I think you can both take more advantage of other masteries and also the time you spend destroying towers is nothing compared to the time you will spend fighting other champions.

This Mastery setup also gets along with the Rune setup, going heavily into Offense. This setup gives a very strong early game and lets you completely dominate your lane if you have Ashe's basics.

Defense Masteries go into Armor and Reducing monsters damage beacuse Ashe's a squishy champ early game and this really helps.

First, Boots or Dorans? I personally like to go for Dorans cause besides de +10AD, they give 80HP wich will come hand if you consider Ashe as a pretty squishy champ. This will also allow you to have more damage = easier to last hit. Ashe has 300 speed wich is the lowest speed at level 1 any champ can get, but dont have to move THAT much at early levels, you can get bersekers at level 4-5 if you're good at last-hitting, so basically you just have to "survive" 4 levels without boots, thats not much.

Speaking of damage, Ashe is a champ that needs crit. Period. Getting Infinity Edge as 1st heavy AD item is a PRIORITY. NO Bloodthirster, NO Black Cleaver, Infinity Edge together with passive lets you hit for 400 AD. And here's some maths that prove it:
Lets say that you have Dorans And Infinity Edge, you should be something around 180-200 AD, but lets take the smaller value, 180. So you have Infinity Edge's passive letting you Crit for 250% damage: 180 x 2.5 = 450. Lets say you finally hit for 350-380 if we take out enemies Armor and Defense stats. 350 damage when enemies will have around 1200-1600 HP is A LOT.

Vampiric Scepter before Infinity? YES. You need that sustain that you dont get from Dorans Lifesteal. Even though you're laning with a healing support (Taric, Soraka, Sona) you can save their mana for ganks and harrasing either than always healing because you're low. Besides, More HP = More harrasing, so you can keep your farm up, while you try and get enemy carry's farm down.

Late game items really are your choice and depend on enemy team as well, but one more thing i would like to mention about this build is that you have no need in rushing Bloodthirster because with Vampiric Scepter + Infinity Edge + Phantom Dancer, your healing should be pretty good.

Rushing Hawkshot vs Rushing Volley or Frost Shot.

When I play Ashe I make sure im gonna farm like mad. When you get level 4-5 of Hawkshot (E), you make sure your income increases by 20-25% due to this awesome passive (not to mention if you're playing with a Twisted Fate). Early game is mainly farming and its here where Ashe takes the advantage. If played properly (farming and not being dead), you should't be afraid of not be getting kills, trust me. Mid and late game YOU WILL OWN. You will get items MUCH faster than the enemy carry and that will make the diferrence from min 12-15 ahead.

Will I lose ganking potential with no or little slow? Yes you will, but getting that Cristal Arrow and jungler coming in with red should be enough. If thats not enough you still have Ghost and/or Flash to secure the kill.

I personally find this Skill Sequence the most beneficial, after trying and playing with Ashe over 150 games.

Ghost and Flash, why? Im just gonna say one thing here: Ghost + Frost Shot = No escaping.

You also get Ghost because YOU ARE GONNA GET TARGETTED if enemy team is just a bit smart. So this 10 second boost lets you escape from someone and at the same time come back into teamfight quick enough so your team doesnt lose the main damage dealer.

Flash is a must in almost every champ and for its deceiving potential I wouldnt take it off.

Maybe you consider Heal, Cleanse or Exhaust as other possibilities, just make sure you talk to your support and agree on what Spells you want in the lane.

Do NOT take:

Promote
Smite
Clarivoyance
Revive
Clarity
Surge
And I personally dont like to get Ignite on AD Carries but thats up to you

"While out of combat, Ashe's Critical Strike chance increases every 3 seconds for her next attack."

To start with, Ashes passive increases 3% her Crit Chance every 3 seconds at early levels. This means that you need 33 seconds to reach a 100% Crit Chance. Considering laning phase starts at 2:00-2:15, even if you're playing top side of the map, you can leash or help blue and charge next Crit.

Crit Chance Increase per levels:

1-3 --> 3%
4-6 --> 6%
7-9 --> 9%
10-12 --> 12%
13-15 --> 15%
16-18 --> 18%

In order to increase your harrass potential, you can walk around charging your passive up to 80%-100%. When this happens, try to hit ad carry with auto attack/volley/auto attack and walk away. This harras should take like 300-350 damage, which is unsustainable to any healer if you do this several times. You can also alternate between AD carry and support to maximize your damage and force support to heal alternatively.

Remember to do this and KEEP FARMING.