A trench coat is a key element of the professional woman’s wardrobe. It is the correct outerwear for suits, dresses and other work-wear when the temperature is between 55 and 70 degrees, and it confers elegance upon casual ensembles including jeans and sneakers. While the words “trench coat” probably conjure a general picture in your mind, the actual execution can vary significantly (through cut, material and length) and so, in a perfect world, you might ultimately own more than one to serve in specialized roles. But before we get ahead of ourselves, let’s discuss First Principles.
3. Your trench coat should be roomy enough to fit snugly over a fitted jacket or thick sweater, but not so big that it looks oversized when you are only wearing one layer (a dress or skirt and blouse) underneath. I am not a fan of the trench coat with a flannel lining that can be removed; it looks bulky when the lining is in and baggy when it is not.
5. The classic colors for a trench coat are beige, putty, khaki, and British tan; surely every skin tone will be flattered by at least one of these choices. Keeping in mind that the trench coat should be worn with casual wear, too — you may want to consider dark blue if you frequently wear khakis. A khaki coat with khaki pants is a lot of khaki, where an inky blue coat looks smart with khakis and jeans. If you have more than one trench coat, black or off-black can also be very beautiful. If your wardrobe is almost exclusively neutral colors, you may consider red or scarlet — but only at a shorter length.
6. The material will dictate how dressy the coat is: cotton twill is generally casual, a crisp poplin or cotton blend (cotton with nylon, elastine or other synthetic fiber that creates a slight sheen) is dressier. Wool gabardine is dressy, beautiful, and expensive.
8. If you buy a light colored trench coat, be certain to have it cleaned regularly and ask your cleaner to pay particular attention to the insides of the collar and cuffs which will get quite dirty. Oil from your hair and neck (precious bodily fluids) as well as hair products and sunscreen can soil the inside of the collar and neck, in particular — so you may want to wear a lightweight silk scarf with your coat (tucked inside the collar) to provide a little extra protection. 
9. When you have worn your trench coat for years, you may find the collar is stained or the cuffs are frayed. Your clever tailor can turn the collar for you and fix the cuffs. Trust me; my clever tailor has done both.
10. Ironically a trench coat doesn’t provide much protection against rain.
The Trench Coats: Burberry Double-Breasted Trench Coat in Ice Pink; JCrew Icon Trench; See by Chloe Jacket in Beige or Steel Grey at YOOX; Marc by Marc Jacobs Classic Cotton Trench at Shopbop; Nanette Lepore Mesmerizing Trench Burberry London Short Tailored Trench in Ink; Burberry Brit Straight Fit Pleated Back Jacket in Ink Blue; Burberry Brit Double-Breasted Trench in Crimson