This is just an idea I had randomly one day. The cornerstone of this build if your trinity force. Utilizing the burst damage of spamming volley in coalition with basic attacks with the sheen proc. I wouldn't recommend this for beginning Ashe players but rather for experienced players looking to put a new spin on an old champ.

Armor pen. Quints and marks, these will increase your damage output significantly throughout the whole game, and gives your volley a very good bite for harassing in your lane.

Mana Regen. Seals, since you're going to be spamming volley as much as possible to pump out major damage and activate your sheen proc., you'll be needing these.

CDR Glyphs, Cooldown reduction is difficult to itemize for ashe, but necessary for this build so you can spam volley.

I use my masteries to maximize mana regen, cooldown reduction, and armor pen.
See runes for explanation.

Pros:
-Fantastic damage late game
-No different than other ashe builds in the laning phase
-Spamming volley instead of scoring critical hits will benefit your team much greater in team fights
-Better harrass than most other Ashe builds
-Of course there's the slowing attacks and awesome ult
-Slightly less squishy than most other Ashe builds

Cons:
-Difficult to play
-Your team will probably rage
-Weak mid game while you work Tri force into the mix
-Mana dependant

As I said, the tri force is the backbone of this build, with volley being a good spam ability you will get plenty of sheen procs, and the movement speed, attack speed, crit chance, health, mana, and chance to slow, are all helpful on ashe. It's really a solid item for her, the tough part is the stage where you're building up to it. Other items are pretty straightforward, maximizing straight damage, a little armor pen, mainly for the early boost, and crit, sheen proc crits endgame can hit for over 1.5k!

Attack Speed Does not scale well with this build, maximize you attack damage to increase the hurt of your volley and sheen procs.

And yes I realize that the yomumu's active only lasts 4 seconds on ranged characters, but the early armor pen from brutalizer, cooldown reduction, and the crit chance are all extremely good for this build.

I prefer last whisper before infinity edge even if nobody on the other team has high armor because towers have such massive armor.

Priorities:
1. Enchanted crystal arrow
2. Volley
3. Frost shot
4. Hawk shot

Max her ult first duh, other than that max volley first to lower the cooldown earlier. And after those 2 I value her slow over hawk shot, it's the reason Ashe can kite anybody for days. The main purpose I find myself using hawk shot for is when I'm ganking and somebody tries to juke into the brush.

Ghost + Flash is, I think the best choice of summoner spells for Ashe regardless of your build. Flash will make you much harder to gank at mid and allow you to escape if things dont go your way in a team fight. And ghost is good for running, chasing, getting to a teamfight that just erupted across the map, it's just a great spell for any champ.

-Your harass combo, once you have your sheen, should be Volley followed up by an auto attack.
-FARM! Another thing volley is good for is farming, this build is expensive!
-Early/Mid game, right around when you've got your Boots, Sheen, and brutalizer, you will be weaker than you would be using most other Ashe builds, but you will still have a great harass! this will require you to adjust your ordinary play style for Ashe!
-The blue golem buff is AMAZING with this build!
-When pushing towers still spam volley whenever possible, the following sheen proc will hit the tower very hard.
-NOT FOR BEGINNING ASHE PLAYERS

-Don't knock it til you've tried it ;D

Try getting zeal, phage, or both first you will be able to spend more game time in your comfort zone seeing that these are more "normal" items for conventional Ashe players. I choose to get Sheen first because that is essentially what this whole build is focused around, and sheens' effect makes for very good harassing. The small mana pool bonuse is also nice to help farm easier with Volley.