Ashe is an AD ranged attacker ideal for both new and old players alike. This build keeps ashe alive while making the most of her passive. So many other builds just get two Phantom Dancer and an Infinity Edge with some Berserker's Greaves and other AD items. They create a powerful AD carry but it costs her survivability and kiting.

Boots of Swiftness allow Ashe to continuously slow her opponents and keep at a safe distance. Ashe is very fragile and melee champions will smash her if they get within range. Ashe is able to keep her opponants back with both these and her movement speed quints.

Since the last patch when they buffed the Cloak and Dagger, it has become an almost essential item for AD carries. Not only is it a great mid game item, but it also gives that life saving +35% tendancy. The tendancy makes slows, stuns, fears, taunts less effective, helping you get back in control of your champion faster. In a lane facing Veigar, Morgana, Lux, Pantheon, Taric or anything with a slow/stun GET THIS. It can mean the difference between getting a kill, and being a kill. Why not Mercury's Treads? Well they're good for a melee champion or a champion with better movement speed, but Ashe has no escape against ganks unless she has her ultimate. Her base movement speed is appalling and getting a cloak and dagger is simply more effective for ashe.

Infinity Edge if you don't get this on ashe, you have some serious problems. Ashe's passive gives her crit chance. make the most of it with this. No further explanation required.

Bloodthirster is a great AD and lifesteal item. It is a must for most AD carries and ashe is no exeption. Keeping it farmed isn't a problem as Ashe is an excellent farmer.

Last Whisper is another great item for AD carries. Some will argue saying Black Cleaver is better but on Ashe Last Whisper is your best bet. Why? Because of Volley which will not activate Black Cleaver. I LOVE Black Cleaver on champions such as Poppy, Pantheon and other AD bursters, but Ashe is more utility and team oriented then these champions. The last whisper is my pick for Ashe, without it she simply doesn't deal enough damage.

Banshee's Veil is the solution to Ashes weakness to magic and CC. Along with your trusty Cloak and Dagger you should be able to escape most fights alive. This item allows Ashe to not be affected by ultimates that would otherwise kill her.

Alternatives to these items include:
Phantom Dancer

Runes:
Greater Mark of Desolation: Provide Ashe with a bit of armour pen for early game, cause her to do more damage. Alternatives are crit, damage or attack speed marks but personally I like these.

Greater Seal of Scaling Mana Regeneration: Keeps Ashe's mana up. Without these you would likely be going to base more often, they allow you to spam Volley and frost arrow to help keep your farm and harass up. Alternatives are armour, attack speed etc.

Greater Glyph of Scaling Magic Resist: These are amazing on any champion, especially Ashe. In the current meta game, either an AD carry, or an AP carry goes mid. These are amazing scaling into late game, they help you survive against heavy nukes and well are just generally amazing. Alternatives are CDR per level/flat but honestly I wouldn't substitute these for anything.

Greater Quintessence of Movement Speed: Ashe starts off with a mere 300 movement speed. These are essential on Ashe as she needs to be agile to escape. Along with her Boots of Swiftness these keep Ashe to get where she needs to.

Masteries are a standard 21/0/9 for AD champions, taking the points in attack speed, damage to minions and especially crit damage. The points giving her exp are essential as Ashe needs to be the same level/out leveling her opponents.