I don't have the answer, but I struggle in this area and I have some thoughts:
Hair: Apply the 2 year rule--if your hair is exactly the same for two years in a row, do something different. Nothing is more dating than a "frozen in time" hairstyle. I met my new doctor this week, and the first thing I thought was, "dated 80s hair." Also, consider coloring your hair. Even before gray hair is an issue, hair can fade.
Jewelery, shoes, bags: Resolve doubt on the side of trendy, and on the side of "early adopter." What I really mean is to make yourself a little bit uncomfortable, instead of safe, with at least one item per day. Little bitty "real" necklaces look dated to me, but interesting costume necklaces, not so much.
Matchy/matchy: Unless you are wearing a suit for work and you need a very formal look, avoid your top and your bottom being made out of the same fabric.
Avoid vintage: I adore vintage looks on people who are obviously too young to have worn them in their day. Over 40, to me, it starts to look like you are simply wearing your own old clothes. Ick.
Show some skin: I don't mean be vulgar, but a scoop neckline isn't as frumpy as a high, tight jewel neckline. As I have aged, turtlenecks are high on my least-flattering list, but a cowl neck that shows some collarbone is still flattering.
Don't wear sweats or fleece unless you are home and freezing to death. Please. And if your sweat shirt has embroidery or pictures on it, please go find a grandma who will give it a good home.