The only technique that will work is to have a poker-faced and it can be done by practice and through meditation, I have seen people who practice yoga and they can have no feeling or expression at all if they want, practice makes perfect if you practice over a thousand rounds and you get to use having no expression at all, you can achieve that if you are dedicated to your craft.
I still don't know what a poker face is, is not having an expression what is meant by a poker face? I didn't know there was this term but anyway, I don't think you should practice yoga because in this case at least you can control yourself, don't panic and can keep your breathing steady. So that way, I'm sure when you do it often you will also be able to apply an expressionless face in poker but still, it seems like it's something difficult in poker because the tension in the game is bound to happen.
Not really, a poker face is simply to not change your behaviors regardless of the hand you are given, so for example if you are a serious player that does not talk a lot on the table and you have a bad hand then you still do not change your betting habits, the way you bet and the time you take to think, basically you need to do everything in the same way despite what you have in your hand, and if you are a talkative and extroverted player then your poker face is to keep being that way regardless of what you have.