Best Method of Long-Term Charm?

[Victim]

Just recast Charm Monsters. Once someone is charmed, the caster can probably persuade them to voluntarily fail saves, especially if they commonly cast spells that are actually helpful. Or you might not even bother if the target's Will save is abyssmal.

It's a bit harder to do with charm person, but a high to mid level caster with extend ought to be able to make a go of it.

 

[Jack Simth]

Just cast Charm Monster regularly. RAW, you know whether or not the spell affected the target. Charm Monster is a day/level spell. Just cast it every day. If it doesn't take on day 1, cast twice on day 2+ until it takes. Then you're on day 1 again. If you exceed day 7, you may have problems, but statistically, you've got a really good chance of keeping them under wraps. Especially if you're an enchanter, and get three castings in on day 3+.

As a side benefit, it's Dispel resistant - there's a lot of copies, and they all have to go down....