At level 14 you can get
Fanion, which has no ASF. I highly recommend this option. Up to that point what I have done is use a crafted light mithral shield, which has no ASF. You can also use augments to reduce your ASF. If you are a crafter there's Lesser Arcane Casting Dexterity as a prefix option that can be put onto rings, trinkets and gloves. There is also gear that lowers ASF, as well as some enhancements. You definitely want to run with zero ASF, which is very achievable. Check out the
Arcane Spell Failure entry on wiki for some good info on the topic.