Try and make your face completely void of emotion. If you can't do this, act the opposite to what you have.
E.g. You have a pair of 2s, but you smile very slightly, this may discourage other players. When you have a very good hand, look uncertan, and then look as if you are playing quite hesitantly.
I was quite good at poker when my friends and I played every week as a hobby, but we gave that up when we got bored of it.
Also, if someone has a REALLY good hand, it is almost impossible for them not to show even the slightest sign of a smile, this can help you judge what they might have.