One of the issues I found running my last campaign is that it is very hard for monsters to successfully cast spells. A beastman shaman or witch doctor may have enough cover from the other beastmen to manage it, but a dragon or sorcerous sphere will often be alone and "win initiative agaisnt 10+ characters or lose your turn" doesn't seem like a good deal for such a powerful creature.
At the same time, I like the riskiness of spellcasting - that spells are powerful, but a spellcaster can be locked down by a determined party.
With this in mind, I'm considering adding the following rule for my next campaign:
For a spellcaster to lose their spell to damage they must either fail a saving throw or take enough damage in one hit equal to or greater than their base HD (ignoring plusses and minuses).
Note that PC almost exclusive come from 1-1, 1 or 1+1 HD races, so for PC casters this replicates the base game rules, but if you want to stop a venrable dragon from casting, be prepared to hit it with at least 20 damage in one hit to stop it.
What do you think?