Thank you so much!
Well, I'm not the best in explaining things (I'm probably not qualified enough either), but I think I can point out 3 things that are most important in my opinion. First - references. Using mirror to see how your own face is behaving in certain expression is also very helpful. Second - maintaining the volume of the muscles. E.g. whenever lips are stretching for a smile, cheeks are growing a little bit. Third - practice. Sounds cliche, but it's very, very important.
Hope it helps you somehow.