Best way to increase charm is to make successful diplomatic proposals to the other factions. What I do is send my princesses to every faction around western europe (I'm english) looking to trade map information and enough money to make the proposal "generous". Just about always accepted and just about always increases the charm rating.

I still get the secret lovers though. Don't know how to avoid that. And I've even had one come of age with a chastity belt from her father, which lowered charm as well.