I keep trying, however, never certain if I have finally conquered the notion. I can't see a campaign without a charm person spell - but damn, a most annoying thing to write rules for.
This post includes my latest attempt. After, the reader may be interested in reading my present rules for hypnotism, fascinate and suggestion. These are all in the same category. Because players do not trust these spells (after years of disappointment), these are largely untested (though the new hypnotism seems to be gaining popularity).
Charm Person & Mammal
Duration: 5 rounds, +3 rounds per level
Area of Effect: 1 creature
Casting Time: 1 round
Saving Throw: negates
Compels the affected creature to protect, appreciate and even become enamoured with the spellcaster, depending upon certain conditions. The spell is more effective against creatures of the caster's race and those who have the potential for seeing the caster as a potential mate than otherwise.
The creature is entitled to a saving throw against magic. If this fails, the immediate concern of the creature will be to protect the caster in a situation of danger. The creature will take a stand between others and the caster and aggressively attack those actively attacking the caster - but the creature will not go forward to attack others not immediately imposing a threat. Throughout this, the creature will express their desire to protect, will speak words to reassure the caster and will make suggestions for where the caster might stand or move in order to make this protection more practical.
The second concern of the creature will be to express their respect and admiration for the caster's person. This will include a willingness to follow along, perform harmless tasks within the creature's ability (so long as they take place within the spell range) and give information when asked.
Finally, if the spellcaster does not actively resist, there is a chance that the creature will fall in love with the caster. For this to happen, the recipient is entitled to save again, with the following modifiers:
Traditionally friendly races for characters would include dwarves, elves, gnomes, half-elves, halflings, half-orcs and humans. Traditionally unfriendly races for characters would be every other race. Note, however, that if a goblin were to cast the spell, a traditionally friendly race would include norkers, hobgoblins and kobalds - but not orcs, lizard men or kenku. Thus it is a matter of what individual of race is actually casting the spell.
For size, apply the categories advanced in edition 3.5. All adjustments are cumulative.
Creatures falling in love will be inclined to remain with the spellcaster after the spell has ended. In special cases, there may be time and inclination to sleep with the caster (it is the DM's call whether or not this includes animals). This requires appropriate, private circumstances where no threat is imminent, then a charisma check on the part of the caster (success indicating a possible willingness) that is followed by a wisdom check on the part of the affected creature (success indicates resitance).
Once the charm has ended, creatures in love who fail their intelligence checks will seek to remain with the spellcaster as friendly followers (long-term love is a tricky, as yet non-ruled game detail) or as protective animals. Such individuals will be have a -4 saving throw vs. further castings of charm person & mammal by the spellcaster. Individuals who have been so affected by the caster will betray signs of jealousy and probable infighting when witnessing affection by others (so that multiple previously affected creatures will be trouble when asked to work together).
All other creatures will react strongly when the spell runs out; as these will retain their memories, the actual facts of what they've just done will greatly influence their actions after their minds are free. Specifics, again, are too wide-ranging to properly be contained by a rule set. Some may merely flee; others will demand explanations; still others will draw weapons, potentially willing to fight to the death or even commit suicide. Circumstances will dictate probable behavior.
Spellcasters are not able to charm persons or mammals with equal or more hit dice/levels than the caster.