1. Don't get too skinny (think Nan Kempner, who looks like a "Best dressed" bag of bones) and don't gain too much weight!
2. Don't wear baggy, shapeless clothes.
3. Don't wear ditsy and fussy prints like tiny florals.
4. Don't wear overdone, fussy styles or details (gold buttons, gold trim, etc.). They are terribly aging.
5. Don't think you have to wear skirt suits to look well dressed. Even though they may be "in," some skirt suits can be very aging. Pants are generally much more youthful, so a better choice would be a pantsuit or a pant and coat ensemble.
6. Don't shy away from all black. It looks great, especially when textures are mixed: leather, crocodile, patent, fur, vinyl (this lightens it up a bit and gives it life).
7. Don't expose your upper arms and neck: these are both tricky areas for most women over a certain age.
8. Don't wear your skirts too short.
9. Don't overdo the grooming. Helmet hair, and too much color -- red nails, red lips, eye shadow and blush (especially when used together) -- are terribly aging.
10. Don't think you have to cut your hair: a chic ponytail always looks smart and youthful.
11. Don't wear fussy, frilly evening attire. Go for tailored instead!
12. Don't wear shawls (too granny) or capes.
13. Don't overdo the glitz! Despite the fact that maximalism is in right now, you don't want to look like a Christmas tree.
In short you will find that many of the rules of dressing your age are common sense and have a lot to do with wearing things that are attractive on.