Watch people with tells and don't do that! Hint, everyone has tells.
Everyone has a rhythm to their game, and everyone gets off that rhythm somewhat when a hand comes along. The players that are toughest to read are the ones that are able to stay on that rhythm when a hand comes along.
It is not important to be any particular way, there is no most effective poker face that works universally for everyone. What's most important with hiding tells is to get into a rhythm that fits your natural personality and state of being, and maintaining that rhythm rather you are in a hand or not.
Be aware of the little things you are doing. Many players with a bigger hand pay more attention then when they plan on folding. Players wanting to bet or raise tend to glance at their chips. Players ready to fold, cap their cards differently, don't glance at their chips, change their body language, shuffle their chips differently Etc. Etc. Watch what they do and don't do that!
One of the simplest processes to get into a rhythm that I use is simply pay attention to the game with the same intensity rather or not I am in a hand. This also has the benefit of picking up a lot of things on players that come in handy to make a profit. I am kind of the quiet type, so it works for me. Some players love to talk, and that can also work well for that type. They just give you more tells then you can possibly sort out accurately, a little like hiding in plain site.