Since this was primarily a Beehive-age problem, we took it to the Mia Maids & Laurels to see if they had any ideas (and of course they did!)
They planned a Mutual Activity in which they (the older girls) would team up with a Beehive and coach them as a sort of mentor in a manners contest.
We originally planned to call this "Princess Academy" but that weirded some of the girls out (we have a lot of sportsy girls who were worried that they would have to wear makeup or be too "girly") so we made sure they knew this was a manners thing and NOT a makeover thing. We changed the name to "Manners for Ladies". I explained that manners are important for everyone, but there are certain things that apply to us as women that don't really apply to the guys (such as sitting properly in a skirt). I talked about how I often have to sit in meetings (like ward council) where I'm in a circle of chairs and I really need to watch how I sit because I don't want to make an awkward situation for anyone across from me.
We made it fun and everyone had a great time. We watched some clips from "Princess Diaries" just for fun (the makeover / manners scenes). We talked about different ways to sit in a skirt and how wearing a short skirt really limits how we can comfortably sit without showing more than we want to.
One of the Mia Maids (wearing pants) came up and demonstrated how NOT to sit in a skirt (she was wearing pants, but we asked everyone to imagine she was wearing a knee-length skirt). She should several variations and it was really funny.
After that, we teamed up all of the older girls with a Beehive or 2, and they went off to different parts of the church to "train" - they learned how to introduce yourself to someone, how to introduce other people to each other, how to shake hands, and (just for fun) how to walk with a book on their head. Then we had all the girls come back in and the adult leaders "tested" them (went around "meeting" them) and then they wanted to walk the "runway" with books on their heads. We declared everyone the "winner!" :)