You gotta let your mind wander and think of something funny, when meeting customers. I once laughed my ass of with a group of customers and it was actually because one of them looked funny, they all were kinda alienated and I said I laughed because of a joke I heard earlier . (You got to be prepared to tell a joke then tho)
Learning facial expression aint too easy, I can understand that. I have no trouble smiling, I am even when looking neutral still kinda smiling, but when I get angry or I dislike something than you can see that in my face too. In fact you can see everything in my face and especially with stupid colleages you dont like or in negotiations this a problem.
Best way to change that is practice. You gotta make yourself aware in public as often as possible how you could be possibly looking at any given moment. And then practice different faces and see how customers react. If many customers smile back to you cause of a certain face, you found your selling face