Mage Armor is a must have. Mystic Theurges are kind've tricky, so to make them work you can't do that many directly offensive spells. The Magical Knack trait is perfect for this. I would focus on buffing and battlefield control. You could pick one class to do offensive stuff with. You definitely want flying, so as to minimize attacks against you. I would pick archetypes that get rid of Familiar, as they will be behind and be a weak link. Speaker of the past archetype plus the Orator feat can help you be a party face as well (Little bonus tidbit there). While the archetype does seem wasted with the revelations, if you primarily go Mystic Theurge you won't really reach them that much anyways. The spell sage archetype for wizards looks quite appealing. The empower spell metamagic feat looks quite nice. Due to the whole level-dependent bit I would focus on buffing hexes. Offensive hexes can become obsolete, but buffing never hurts. Good luck. It takes courage to try out this PrC.
Since you are potentially MAD for casting stat, and also lower level spells compared to a single class, you probably should focus one side as offense, and the other non-offense.
By that, I mean, a high stat for the offensive casting, and a mediocre stat for the other casting, since the latter's saves won't matter so much.
/cevah
Ever since the SLA FAQ nerf, Mystic Theurge has been really crippled. You would be better off searching these messageboards for Shaman builds that do thte Swiss Army Knife thing without multiclassing.