Chris,
You are so right. Everything you have suggested is absolutely true. I haven't been an artist as long as you have, but 12 years is enough to know that being kind to the customer, helping her get the right piece (I'm a jewelry artist) that really looks nice on the person (and not just to sell the piece), and looking clean and nicely dressed is the key to sell my work. Most of the time they come back and say "I get so many compliments on the piece, I need to get me another one!"
I always try to find outfits that will give me enough neck space to wear my necklaces and jewelry and show how nice it looks with a summer dress or jeans with a t-shirt, since the first thing people say to me when they see my work is "but this is too fancy, where am I going to where that to?" I tell them....to the movies, to dinner, to go out with your friends! You don't need any fancy dress to wear these pieces. It never fails....I sell at least one of the sets I'm wearing at each show.
I don't invest tons of money in clothes, but I try to find summer dresses and nice tops to actually sell my work! Make up and hair is a must for me, too.
I do wear COMFORTABLE SHOES...because, as you said, I spend most of the time standing up and talking to people. Gosh...I've seen artist that look like homeless, seriously. I don't even feel like getting in their booth when I see the way they look.
Thanks, Chris. It's funny because I had this conversation with one of my fellow artist and we discussed everything you said in here and totally agreed.

Happy sales!