I wonder if you have seen yourself on video? Or been told your movements are not sharp enough? You need to avoid the "cat shaking a rat" type of movement, make your stops and turns crisp and well defined (don't overshoot and pull back to try to improve the sharpness). I think a lot of the sharpness in tango movements is an illusion in the viewers eyes, their brain expects you to continue in the direction you are moving, a sudden stop or turn will register very sharply. Remember the shot towards the end of "The Graduate" movie where Benjamin is running towards the camera on a long telephoto lens? When he changed direction and turned right into the church path, the effect was amazing.
Facial expression should also be natural, just a bit serious as the dance is a play between the man and woman. I was told your eyes never meet as you would typically just spent 6 months on the ranch and not brushed your teeth during that time!
Remember the lady has to follow the man, so don't bruise her!