I preserve my smile.
In the early days of my illness I would walk into my clinic and smile at everyone like I usually did. Within a short period I looked very sad and couldn't smile any more.
I learned to smile briefly and than stop. This way I could always smile at someone again.
People don't look at how long you smile at them, they just notice if you smile or not.
Although, there are times when I am sure I am smiling and then realize that I look like a sad puppy. (if someone happens to take a picture at that time).
And there are times when people ask me why I am sad (when I am not).
But, there are many more times in which I have a perfectly normal smile (even if for a brief period, but no one knows).