I main Ahri, but pretty much what Basoosh said is right on. To add some things I noticed:
Only use your ult if you know you will either A) get the kill 100% or B) as an escape from death. Ahri without her ult makes her very vulnerable. I tend not to use it unless I meet these two criterias.
A few times you get too close to the enemy. Make space between you and the enemy. Utilize Ahri's spell range. Try to stay as far away distance as possible. If you are too close, you can get hit by stuns, knockback, etc, and may be focused on.
You should upgrade your boots sooner. Ahri is already slow as is and every bit of movement speed helps her, plus magic pen will give her extra damage.
No need to have 2 or more people capping one turret at the same time. If you find you are capping and your teammate comes and caps it too, just leave and go heal/help your other allies. I usually end up with the lowest points on my team, but I'm always mobile and assisting teammates.
As for summoner spells, Ahri does not like to be immobilized, so I like to run cleanse/ghost on Ahri. I found these two to be the most effective on her. With cleanse, you will be able to dodge any stun/taunts on you then ult away to safety. I can't tell you how many times cleanse has saved me from ganks or deaths. Ghost is just good all around plus its an extra escape mechanism when your ult is down. Flash is not that great in Dominion, as almost everyone has some sort of gap closing skill. And ignite = getting close to enemies to use, which is not what you want to do, so not a good spell on Ahri.
I'm not sure if this was going to be in your build or not, but I find Rylai to be very good on Ahri. She becomes a kiting monster with it. Just cast fox-fire and watch it slow everyone trying to go after you. Plus it makes it easier for you to aim your skill shot spells.