A poker face is very hard to keep up for hours and hours of playing.
Just show no emotion if you're going to do that. People who have been in that game and managed to get to show no emotion, they know how to do it and can sit for a long time without showing what they feel.
There are quite a few people on the table who observe any expression of your face and even look at your throat how heavily you are breathing. When playing serious for money I would recommend to wear sunglasses, a hoodie and. a scarf. This makes sure that other people can't read you that easily. Being unpredictable also helps, as long as you are playing good poker.
Yeah, it's always one factor that other players look at but it's not just all about having a poker face but you can also fake a smile or disappointment in front of them.