So after living in San Francisco for half my life, I've picked up a couple tricks/guidelines to keep my weekend outfit practical yet stylish and effortless. The trick is to layer, but keep the volumn down. Layering with thin pieces allows you to play with more elements (color, prints, pattern...etc) and this way you won't look you haven't yet shed those extra "holiday-pounds". Mixing prints and patterns might sound daunting, but it's might rewarding at the same time; it's mostly about trial and error.
I find that the easiest way to look both put-together and young is to keep it tamed but interesting at the same time. A crisp white collared shirt layered with a bright solid v-neck sweater will do. The v-neck and the open collar of the white shirts frame your face nicely while the contrast between the white and the color of the sweater keeps the look fun-loving. To keep yourself warm, throw on a thicker outerwear (by keeping the other layers thinner, you now have the liberty to actually wear a thick jacket without looking like it's the 10 millionth layer on you; my go-to item is a stiff-collar black pea coat from H&M). Make sure the coat is around knee-length to keep the proportion in balance, nothing says elegance more so than a mid-length coat.
Throw on a pair of jeans or slim-cut khaki along with your favorite pair of sneaker and you're SF weekend ready.