I start out with thevampiric scepter because of it's Life steal. it's a great way to heal Ashe and her squishyness. If you spam her Volley on champs only and use the life steal on minions ultimately you can get her Ice Arrow to finish what's left of the champ early game.
Black Cleaver should always be an early game item. Black Cleaver allows Ashe to cause great damage to tanks as they build up armor.
Cloak and Dagger is necessary in order to bring out Ashe's Crit strikes. Using C&D, Ashe crits minions on every 2 hits. Ashe crits on champions more frequently as their health drops.I personally use this item is because of my extreme crit damage. This item also has tenacity. It reduces the slows, stuns and traps that Ashe is often confronted with. This item helps to protect her from the slows. It has a 20% crit damage increase. This in combination with your crit runes makes for a hard hitting Ashe.
Next Is the movement. I never focus too much on it since I have my ult and ghost. I do this so that I can focus more on my Infinity Edge. However, Berserker's Greeves can be helpful as well seeing how Atma's and Black Cleaver will be sufficient.
Infinity Edge adds AD to the crit damage that is inflicted on the other champs. This is a Key item for Ashe. Ashe is build mostly AD even though she does have some magic in the shape of her arrow.
Phantom Dancer is a great way to increase her attack speed which sends more arrows to enemies. This increases her movement speed which she greatly needs. Her movement is so slow that she needs the extra boost.
Hextech Gunblade adds AP, Life steal and Spell Vamp. The life steal helps keep her alive and adds damage with her Ice Arrow. The active is a good boost to her attack damage as well.

I mostly Lane and find lanes that have pushed to our turrets and push those back to their turrets and move on to the next lane that needs pushing back.

Occasionally around level 11 while possessing Black Cleaver and Atma's I pick up the golems at bot/top, the banshee's at mid, the wolves. as I level to 15 I do dragon solo. around 17 or 18 Ashe is able to partake in Baron with the help of 1-2 others.

I particularly like to Solo when I can. It advances her levels to be more effective while also building her quicker.

Frost Shot
Toggle: Ashe's basic attacks slow her target's movement speed by 15 / 20 / 25 / 30 / 35% for 2 seconds.

By following the skill sequence above, I tend to not have a problem in slowing targets from escaping. However, depending on how the game is going I tend to learn heavily on Volley.

Volley
Ashe fires 7 arrows in a cone dealing attack damage + 40 / 50 / 60 / 70 / 80. Volley also applies Frost Shot.

If the other team is heavily pushing against my turret, I level up volley a lot quicker than frost shot. This does greater damage to minions as you clear those to prevent attacks. Leveling Volley also helps with CD Reduction. This lets you spam her abilities more effectively.
Hawkshot
Passive: Ashe gains 1 / 2 / 3 / 4 / 5 extra gold whenever she kills any unit.
Active: Ashe animates a Hawk to scout for her, revealing terrain as it flies toward target location.

I take Hawk Shot as my 3rd ability in all lanes but especially in mid. It helps prevent ganks and it also gives a gold boost. However the gold incentive isn't a great boost to your income so I don't level it as quick as the others.
Enchanted Crystal Arrow
Fires a large arrow in a straight line. If it hits an enemy champion, it will stun that champion for up to 3.5 seconds, based on the distance the arrow traveled, and deal 250 / 425 / 600 (+100% of ability power) magic damage. Additionally, surrounding units take half the damage and are slowed.

The CD on the arrow is rather quick as it levels. I tend to fire my arrow often at other lanes and great distance shots to assist in kills. This gains me Assist Gold and helps to assists in starting ganks.

You want armor penetration and your critical damage. those are the strongest aspects of Ashe is her huge crit's and needing the armor penetration to take down the tanks.

Stats on runes:
+12% Crit Damage
+25 Armor Pen

Rune Breakdown9 Mark's of Desolation3 Quint's of Desolation9 Glyphs of Furor9 Seals of Furor