I normally think like this: an actress is a female actor. If you only get one word to describe her and her job, and if it matters to communicate her gender, then it's "actress". But in a sentence where her gender is clear--or in cases where it's irrelevent--I use "actor".
"Who's your favorite actress?" is a legitimate usage. "Liv Tyler is the finest actor of her generation," is also a legitimate usage, though arguable.
That's a for-instance. I use the same rule with "waiter" and other like role terms.