In the last post I mentioned that mages should be hit capped and I realised that there would be people out there who wouldn't be sure what I meant by this. I felt the need to explain.
As a spell caster, all mobs have a chance to resist your spells, spell hit is the stat that reduces the chance that enemy targets will resist your spells. No matter how much spell hit you have, all targets will always have a 1% chance to resist your spells, this can't be negated, removed or avoided in any way. Pulled from WoWWiki here are the base numbers for PvE targets. (PvP hit is calcualted differently when players are more than 2 levels above you)
Level Difference= -3 Chance to hit= 99%
Level Difference= -2 Chance to hit= 98%
Level Difference= -1 Chance to hit= 97%
Level Difference= 0 Chance to hit= 96%
Level Difference= 1 Chance to hit= 95%
Level Difference= 2 Chance to hit= 94%
Level Difference= 3 Chance to hit= 83%
For PvP at level 70 you will never need more than 3% hit, heroic bosses are considered to be level 72 so for heroics you will only ever need 5% hit. When you move onto raid bosses spell hit becomes much more important and you will want to reach +16% chance to hit (202 spell hit rating)
For mages there are two talents that increase your chance to hit with spells. Elemental Precision on the first tier of the frost tree increases your chance to hit by 1% per talent point with fire and frost spells, up to 3% with 3 talent points spent. All raiding fire and frost mages should have these talent points and will be looking to get 13% spell hit from gear (164 spell hit rating). Arcane Focus from the first tier of the arcane tree increases chance to hit with arcane spells by 2% per talent point up to 10% with 5 talent points spent. People speccing for Arcane Concentration will usually want to put points in here to get there, these points are helpful when sheeping or spell stealing. With 5 points in arcane focus, only 6% hit is needed to be hit capped with arcane spells, however most arcane mages will be using fire or frost spells as well, so more spell hit may be needed anyway (I'm not really sure how raiding as an arcane hybrid works to be honest, but you want as much hit as possible with all your spells). If you are in a group with a draenei mage, shaman or priest, and know you will be within 30 yards of them you gain an additional 1% hit from Inspiring Presence. Elemental shaman have the talent Totem of Wrath that will add an additional 3% to hit, in situations where you know you will be gaining these affects you may want to switch your gear to add additional crit or damage. On fights such as Gruul, you may at times be too far away from people in your party to rely on these abilities for the whole fight, only reduce your spell hit accordingly if you are likely to have these effects for the whole fight.
Spell hit is not to be confused with spell penetration, spell penetration is predominantly a PvP stat, every point of spell penetration reduces your target's resistance to your spells by one point. Useful in PvP for negating the effects of a targets Mark of the Wild or any other similar buff. However in PvE almost all raid bosses have no additional resistance apart from a base +15 to all schools of magic that is not negatable by spell penetration, they gain this due to their difference in level. This accounts for any partial resists you may see in your combat log. There may be excepetions such as Supremus who is thought to have around 200-250 fire resistance, but for the most part spell penetration has no use in PvE. For this reason mages should be running with a Subtlety enchant rather than the spell penetration enchant that a lot of people seem to have.
Increasing your spell hit up to the hit cap will increase your dps more than adding additional crit, haste or damage.
To reach the hit cap you can enchant your gloves, add +spell hit gems to your gear, or look here for a list of items mages can use with +hit on them.
So now you can make sure you always hit your target! (Except for the unavoidable 1%)