For arcane, the best rotation is simply arcane blast until you run out of mana. If you're at around 20k mana, this should be enough for any normal fight, and for heroic bosses, should also be plenty as long as you actually use your gems/evocation. If you're raiding (where mana becomes a concern), go ahead and do as above and figure out an actual rotation, but otherwise just cast arcane blast - it's the best dps you can do.