One of the most important external skills for actors to learn is how to memorize and execute blocking flawlessly. Blocking is a theater term used to indicate the movements of the actors in the space provided for the performance, including their entrances and exits and where they should be positioned for each scene. A great blocking tip is to take the time to pace through the movements, even when not practicing your lines. Memorizing notes is of limited use. If you walk through it, it helps ingrain the movements.
When an actor is on stage, he needs to remain aware of the audience at all times. The problem is that this is often required while you are dodging around a complicated set and many other actors. It's not necessary to face the audience at all times; in fact, that often looks unnatural and forced. Make notes in your script as to when the focus is supposed to be on you. Keep this as a subtle reminder of when you absolutely must be in your light and visible to the audience, barring specific direction to the contrary.
One of the worst things actors do is indicate in an obvious way the things that are happening to them, unless they are participating in an extremely stylized play. Crossing your arms when angry, grabbing your head to indicate it hurts or holding up a hand to your mouth in an exaggerated yawn, unless explicitly called for by the director, does not gracefully communicate to the audience what is happening. Instead, it comes off as an exaggerated and unrealistic reaction. Concentrate on real gestures, not overblown indicators, to convey a truthful and convincing story.
The one thing an actor needs externally more than any other is precision. An actor in total control of her body can communicate everything she needs to. For instance, if playing a character with a limp, the entire body needs to support the limp, as a true limp changes every aspect of a person's movement and posture. An actor needs to be a master of observation, and to train her body to replicate exactly what she observes.