Here are a few optional rules for conditional spell penalties. These are just a taste - you can always add more. And since I need to say this explicitly in either the post on in the comments after I get asked - these are not currently in force in my DF game.
This is at best a rule idea and rough draft.
Resisting a spell that enforces an existing condition is more difficult. For example, it's harder to resist fear effects when already afraid, sleep spells when sleeping, loyalty spells when already friendly to the caster, etc. In such cases, resistance is at -1 to -3. Failure (or successful resistance) doesn't undo the existing state; critical success or critical failure will.
Conversely, it's easier to resist a spell that is explicitly and diametrically opposed to you or your actions. For example, a character with Cowardice being subject to a bravery-type spell, a mortal enemy being subject to a loyalty or charm spell, a spell to forget directed at a deeply-seated memory, and so on. In such cases, resistance is at +1 to +3.
Here are some examples:
Sometimes people want to ensure a sleeping guard stays asleep. A GM can rule as follows.
Sleep - A sleeping or dozing subject resists Sleep at -3; successful resistance means the subject continues to sleep normally. Critical spell resistance or critical failure on the spell means the subject wakes up.
Forgetfullness - Characters with Absent-Minded resist this spell at -3; recent memories (within the last minute of the casting) can be forgotten more easily - resist at -2 in these cases.
Here are cases of opposition:
Bravery - Subjects with Cowardice resist at +3! Conversely, those with Berserk resist at -3 - the spell is pushing them in the direction of their normal course of actions and expanding it.
Lure - Subjects with Incurious or Single-Minded (if they're working on an unrelated task) resist at +3.
This has to be solid opposition or solid sympathy - merely wanting or not wanting the result isn't sufficient. Spells already assume "adventuring" conditions - Sleep spells aren't resisted at +3 because a fighter is swinging away in combat (the normal use), but might resist at +3 if he woke up a short time again after a full night's sleep and downed an energy drink. As you can see, this mostly makes sense with Mind Control college spells - you don't take more damage from a fire spell because you're on fire already, or get levitated more easily because you're mid-jump. It also doesn't make game balance sense to allow, say, low-ST characters suffer more from weakness spells or low-HT creatures to suffer more from HT-resisted spells. That's already occurring - it's a conditional bonus or penalty based on basic sympathy or opposition, not a feedback loop of punishment or reward.
Conditional modifiers that already exist supersede these - the +5 for Absolute Direction against Disorient, for example.