Priests. Why we are so ..umm.. difficult.

[mjdoom]

Actually, something I've learned having two mages behind Aleri frequently is to not shield before the AoE.  Let the mage take one or two hits (depending upon how hard the critters hit of course and how many critters there are), then shield, slap on a renew and be ready for a flash heal if shield wears off before the soul sickness does or before all the critters are dead.  In really nasty situations, that actually keeps the mage alive better than shielding beforehand.  At least if you have renew talented up as much as possible, it does. 

If your mage is prone to panic or doesn't have improved arcane explosion maxed then this method may not work quite as well, but for me and my frequently-grouped-with mages, this keeps them alive and uses less of my mana and less concentration and worry than the way you describe.
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Hmm, in my experience I always shield prior to aoe if there will be enough mobs that hit hard enough (experience helps here with how hard mobs can hit). In some late game cases mobs can tear through a max rank shield in a matter of 2 or 3 seconds. This is barely enough time to slap on a renew and cast one flash heal. In these cases your mage with aggro is taking damage faster than you can possibly flash it back. If the mobs will not be dead within about 10 seconds then you will want to be able to shield twice.

If you shield 5-10 seconds prior to the pull this will ensure that weakened soul will wear off by the time you need to reshield. It will also allow some time to regenerate some of that mana before the actual pull so it doesn't hamper your mana efficiency as much. I will admit that the times that this is a necessary tactic are few but it's still notable. Remember that once your mage has aggro that is pretty much permanent and it becomes a kill or be killed situation. The only reliable ways to save some hits from the mage are to: 1) Heal him or her 2)Kill all of the mobs 3) taunt one off temporarily (this can save one or two hits at best from one mob) 4) have a warlock aoeing at the same time. As has been hashed out in the mage comment thread a warlock can certainly pump out more aoe dps in a controlled situation and this can transfer aggro off of the mage.

Preshielding is certainly not always necessary, but there are also definitely situations where it is and I'm not sure that it's not a good habit to get into anyway. Any situation where you are preshielding is probably a controlled pull where you are at or near max mana on the pull. In that case any mana you have will be concentrated on keeping the primary AOEer alive and generally the mobs will be dead long before you go OOM. The extra buffer added by the preshield is certainly worth it IMHO.

- mjdoom

Edit: I don't want to sound contradictory but you made the claim that your method "keeps the mage alive better." Where is your basis for this statement? As I see it you are leaving less margin for error (with weakened soul) and intentionally letting the mage take more damage, what am I missing? (This is an honest question, please don't take it the wrong way)