Here are my 2 cents. btw I am Arcane (40/0/21)

I usually dump my mana pool during the 1st minute. Pop acrane power, PoM, Icy Veins, Trinket and AB spam till oom(for fights that are agro sensitive, i will evocate at arond 60~70% mana pool to top it up and put my Evocate on early cooldown).
Evocate up. Then cycle 3x AB and 3x Frostbolt, pop mana stones and pots when ever can. This way, b4 the fight is over u can evocate again, and spam AB till oom, and the boss should be dead by then.

I dun usually have a spriest or shammy in my grp so this way i usually dun hav mana issue. The only encounter where I will hav mana problems is during Al'ar.

For Hydross, I dun usually AoE, locks do a better job, just focus on hydross, and throw a frostnova during nature phase, or a flamestrike during frost, and continue to dps the hydross.

Not sure if this is a normal practice, most of the mages in my guild will conjure all ranks of mana stones and use them during the fight, but for me, after i use up my mana emerald, i will re-conjure it during the fight .

My armory
http://www.wowarmory.com/character-sheet.x...Remar&n=Shykull