I spec arcane, currently with T4 gear, 43/15/3 with about 11500 mana (with AI but before GoTW and kings buff). I join my guild in raids only recently and not the top damage yet because my guildmates outgear me at the moment, but i am around the top 35%.
The way I see it: better finish a fight with no mana, no gems, potion and evocation on cooldown than having a half-full mana bar which is of no use anway. Gold is only an online currency and since I am paying the monthly subscrition, better enjoy the game than trying to see how much gold I can save on consumables.
For thrash and small minor bosses, I just spam AB. The trick is to consume a gem when your mana reaches 80%, so that the gem tops it up to full. Do the same with mana potion, continue to spam AB until mana runs out. By now, most if not all thrash mobs will be dead. If it is a mini-boss, step back and cast evocation, return to combat spaming AB again. When i run out of mana again, the cooldown for gem and potion should be over, so just take them and continue casting. There are few ocassions bosses won't fall under this continuous spanking.
In cases where I know we can't burn the boss down in such direct manner, just rotate AB with fireball and fireblast, or ice bolt, but maintain the same gem/potion consumption and evocation guide. With a shadow priest and mage armour and some movement of the entire raid during different stages of the fight, the only time I run out of mana is when the boss is dead or 1% left in his health.
By the way, if your group can run out of mana, spells and item cooldown etc and still can't take down the boss, you are either undergeared or using the wrong tactic.
Edited, May 12th 2008 11:36am by whitegreen