This is not a usual Ashe build. This is more structured on getting assists rather than kills. If you get a kill with this, good job. You have exceeded expectations. Like my Karma build, this is simply how I play. Again, I'm not level 30 so, no bashing for not being able to thoroughly test the build.

Your job is to run around semi-baiting the other team and slowing them way more than they want to be. Firing off your ulti only to escape or stop someone else form escaping.

You want to start off with a Meki Pendant to combat having to recall too much. Stay out in lane long enough to buy Boots of Swiftness and then haul *** back to your lane. After that you want to work on buying a Phage and then repeat until you can buy a Frozen Mallet. You now have a permanent level 4 Frost Shot for no mana. At some point around here, you sell your Meki Pendant and buy a Philosopher's Stone Philosopher's Stone to help you stay alive and get even more mana. You want to buy 2 Phantom Dancers so you can run away/toward them and crit to kill minions. The Shurelya's Reverie is a little bit more HP and mana regen. And also to get rid of your Philosopher's Stone Philosopher's Stone for something better.
The last item you get is situational.
VS Karthus: BUY A BANSHE'S VEIL!
Same goes for massive CC-ers
VS AD like Master Yi: you run and buy a Frozen Heart.
Anything else... Use your best judgement.

You want to be using to harass and get last hits on minions as much as possible. you should not have mana problems until you get to around level 8 when you can really spam your Volley. You only want to take one point into Hawk Shot so you get the extra gold and can scout bushes for ganks and safe places to recall. Late game you want to be using Hawk Shot to see if the other team is vulnerable to gank or going for a large buff like Baron.Frost Shot should only be used while in a team fight. If you are really lucky/good with aiming your Ult, you can use it to delay a fight at your turret after spawing/recalling. Often, I see Champions retreat simply because I hit them with my arrow, because they don't know how close I am or if my team is waiting in the bushes.

I use the attack speed runes so I'm slowing as fast as possible and as much as possible.
The mana regen is because she really runs out of mana way too much.
She needs the Cool Down Reduction runes so she has a nice ability to spam Volley.
End discussion.

By laning partners, I mean the people you are laning against. You should always be taking mid. There are some choices for lane partners if you are forced to the side lanes.
Good Partners:
Anivia is a nice laning partner because her Frost Bite Frost Bite works so well with your Frost Shot/Volley to do a ton of damage early on.

Bad people to lane against:
Vladimir, Fiddlesticks, Karthus. Pretty much anyone who has long range is on this list.
People who have range who you shouldn't worry about: I think Sivir, but I could be wrong.
People who don't have range you need to worry about: Tryndamyre, Master Yi, Taric, Sion... The list goes on. Just avoid anyone who isn't in a team fight or you don't have a significant advantage over.
Be extremely carful of these champions: Rammus, Karthus... I'm sure there are others but the memories just won't come to me.

She needs the Attack speed and cool down reduction. Cripple Helps her chase like a MOFO. The HP and mana regen help her stay out in lane. The extra EXP gain helps her out level, like she should. The extra damage is just nice. She needs the extra cool down reduction on heal. That's all I got. I didn't feel like writing this section. That's why it looks so bad.

I use Exhaust and Heal as a means of staying around and increasing my killing/running power. If you want you can change the masteries to use ghost instead of exhaust. If you do that, take the point out of Good hands and put it into the ghost mastery that I'm too lazy to look up the correct name of. The point you took out of Cripple, put into Archmage's Savvy.
Other options for spells are: Ghost, Teleport, Clarity or, Cleanse.
Don't ever take Clairvoyence. You already have bush awareness with Hawk Shot, you really shouldn't need more.

You really shouldn't but, if you can take the Lizard buff, by all means, take it. Especially if it means kill stealing it from the other team's jungler. It helps you chase even better than you really even should. If you can get someone who will do most of the work on the blue golem and let you have it, do it, it's a great short term resource for Ashe. When the team is going for Baron, you be one of the last people to atack and keep Frost Shot on most of the time. If youre team doesn't have wards, before you start, scout with Hawk Shot down the river and be prepared at all times for other enemies.

Again, this is not the ideal buid, but it is how I play and, unless your entire team is completely feeders or idiots, you should be able to at least hold off 2-3 enemies coming at you and kill at least one of them. This is very unclean right now. I'll be back when I'm in the mood to clean it up to, you know, clean it up...