Ashe can be a decent AD carry if you build her right. Currently, the most popular build on mobafire for Ashe does not give her the right items, and I am very sure that it suffers because of this.
Ashe is intended to do a ton of damage, while the enemies are not looking at her. She is naturally gifted at CRITICAL STRIKES, so make sure you give her plenty of opportunities and items to crit on the enemies.
Ashe is squishy so you need to find a way to keep her alive. She does lots of damage, so a great way to do this is to KILL THE ENEMY. With her proclivity for damage, items that make her critical strike alot give her a solid core of damage. Giving her a way to benefit on that solid core of damage, such as life steal, is a great way to synergize the entire build. By using her damage to fuel her entire character, you maximize her effectiveness as an AD character, and as a survivor.
When you finish the build, you NEED to remember that with it, you can often survive through a half-***ed enemy attack by simply attacking the spellcasters and assassins, which kills them and heals you really fast, then swapping over to the fighters, then tanks.
Lastly, accept the fact that you will die. If you are Ashe, you will never survive being team-ganked, so farming alone is a risky business that can get you killed, because Ashe cannot outrun a team, nor can she solo a whole team. My solution is to use the teleport summoner spell, which will give you a chance to escape or at least waste the enemy's time and give you some more minion kills. It also controls the enemy's movements. Your team can use you as bait for a team gank, and then you can teleport away and take a tower or farm, if you can't help them gank.
Because it is so easy to miss Ashe's ult, I consider Ashe to be ridiculously under-powered in both team fights and last-hitting with it. The stun is practically useless in all but the most unlikely circumstances, but sometimes it can be a big help. In my second game playing as Ashe, my ult won us the game, because I stunned the enemies in a team fight and my allies, almost dead, cleaned them up, then we pushed bottom and won.
Compared to Lux's ult, Ashe's ult is an extreme skill shot, because if it goes a little off target, it completely misses. Compared to Draven's and Ezreal's ults, Ashe's ult is extremely limited in its damage effects. It hits ONE enemy and does small damage to other enemies who are very close to that enemy. Its stun effect is useless at close range, as well.
The skill sprite/animation looks a lot bigger than it really is, so you're effectively aiming a twig, instead of a fat arrow like it looks.
It is better to use her ult from a medium distance*, which stuns the enemies in a group for a short time. This gives your team time to surround the enemies and get a few hits or focus a problematic enemy. During this time, you can use the few seconds where the enemy is stunned to run up and join the fight.
*A medium distance is from your inner tower to just beyond the outer tower.
Last hitting with your ult is extremely wasteful of its full teamwork potential, so insert it in the middle of fights.
You can get some interesting results with your ult if you combine it with the summoner spell, Teleport. If you're good, you can defend a tower by ulting from across the map, then teleporting to the tower, and mowing your enemies down before, during, and after your ult stuns them, all with the help of a tower. It works better if you have a team, but it's possible to do it by yourself, if you get the right items. Do not try to defend a tower, if the enemies can kill the tower in a few seconds, though, because that means you're their next EASY target, unless you already have your high critical chance and life steal.
Make sure you buy critical strike as soon as possible.
Ashe's critical strike chance is very useful for harassing and farming. Combined with the Attack Speed runes, she gets "in the zone" and has more opportunities to land critical strikes.
Even if you're doing poorly or getting focused by the enemies, or even suffering from 1-second lag or frequent disconnects, this item build should give you a reasonable chance of helping your team out, staying in the game, and getting assists, even if you can't get kills.
Buying the Executioner's calling is mandatory against a heal-heavy team, because I have chosen to not use Ignite. Executioner's active applies a grievous wound, which allows you to mow down a champion like Sion who heals all his health in like 5 hits.
There are other viable items, including the Bloodthirster, but I found that I was doing more damage with this build against another Ashe who had three Bloodthirsters. I almost killed him after he ulted me in his base and started attacking. I didn't think fast enough to start autoattacking him, but I almost took out his whole health, when I did start attacking. My team was away, so I wouldn't have lasted much longer against their team as they spawned.
Remember, if you're playing against a particularly good assassin team, Twitch in particular, you really need to buy the Oracle's elixir when he/she/it starts killing you in three hits.
I wait to get my infinity edge, because it is so expensive. It does help Ashe alot, but you can get better results and better farming by going with supplementary critical strike chance items early.
I avoid the last Whisper, because it does not help Ashe survive, nor does it help her team that much. Ashe's primary role is to give her team a chance to catch up to the target. Ashe is good at taking out the enemy's spell casters and assassins that have lower health and lower armor. Ashe generally cannot solo a good tank, so don't buy items for that. Learn to "disappear" and then show up to kill enemies during a team fight.