When an enemy magician uses Conjuring/Controlling to try and wrest a spirit from you, you get to make an opposed test (with bonus dice for 'being there first') to keep your spirit.
When an enemy magician uses Sorcery/Dispelling to try and eliminate your successes on a sustained spell... ... you get to do nothing but watch your spell be whittled down, turn after turn? For things like Shapechange or Invisibility, that could be rather disastrous (eg losing threshold and stopping being a rat while escaping through some narrow shaft, morphing back into full-sized troll mage... ouch!) As far as I can tell, Spell Defence on yourself or Shielding or other such buffs don't provide any way to reinforce a spell currently active on you.
One solution would be to custom-design all your spells to have a very high casting TN so a dispeller is unlikely to be able to take away your successes in a hurry. Then it takes several attempts before you can cast your own spell, every time. A less effective solution might be to design your spells with severely ramped-up drain codes, so although it hurts to cast your own spells, an unwary dispeller might well succeed in cancelling your spell but then has to immediately face an unexpected +6D drain.
Apart from simply dropping and recasting your spell once you're worried about the number of successes you have left (which wouldn't really be an option for the troll rat) what (if any) other ways around this have you tried? I'm aware of quickening/anchoring, and dropping F*2 karma into making the spell harder to dispell, but defending your spirits is an option that comes 'free'... where's the corresponding way of defending your spells?