1. Layers - One of my favorite things to do is to take a fitted long-sleeved shirt from J. Crew and layer it under my T-shirts, camisoles, and other summer tops. This will allow you to get a lot of mileage out of some of your summer staples. I particularly like J. Crew because of the super soft cotton and the snug fit which does not add bulk. Be creative and unconventional. I think that a crisp white button-down works well layered underneath dressy tube tops, strapless fitted dresses, and structured bustiers.
2. Accessories - Make liberal use of warm weather accessories. I know it's a cliche, but accessories make all the difference! They give you more options and a chance to be creative. This is the time of year when I bring all of my scarves, hats, tights, boots, gloves, and legwarmers out of storage. Below I added a printed scarf, some gray tights, and some suede boots to this shirtdress to take it from summer to fall. When the weather gets even colder, I could throw a belted wool peacoat on top.
3. Outerwear - This one is obvious. Peacoats and trench coats are two great classic styles to have in your wardrobe, as they are versatile and never go out of style. Here in southern California, I won't be needing anything as hefty as a down jacket, but lightweight jackets and sweaters come in handy. They also work for cool spring days. Below I paired a vintage Members Only Jacket over my H&M dress for a cool, casual look. In colder weather, I could add black tights, skinny jeans, or a faux fur scarf.