Cherokee, maybe you can nod, say yes when appropriate, or ahem. Sometimes it is just a matter of saying back what they said to you. Just a simple acknowledgement that you heard her is good.
For me, I use to be involved with customers. Most of the time I would not smile. It wasn't that they were upsetting me, usually, just that when I'm concentrating heavily or supremely busy, it just wouldn't occur to me to smile. Especially if they didn't smile at me. Can't tell you how many times I was talked to about that over the years. I guess my concentrating or trying to get things done quickly, looked like a frown to most of my bosses. I never treated the customers with disrespect though.
I also have a tendency to look away when being talked to or when I am talking. Not always, just depends on what it is. I know in my therapy sessions, especially, I look away. If I didn't, I would start thinking about the way she is dressed, how she looks, etc. It's very distracting, so I look away to concentrate.
Most the time though, people say I hold a lot of expression in my face. Unfortunately, it may not always be when I want it to show.