A huge fan. A black v-neck sweater, whether merino wool, cashmere, or a blend- but it must be slim fitting! I saw it pulled off very gracefully and beautifully on a somewhat recent episode of Mad Men and I have been hooked ever since.
The key pivotal ingredient is the white button collar dress shirt and a nice pair of patterned black and white or grey slacks. It also works well with a black and white gingham shirt (i have one in mind, and in my closet, from Shipley and Halmos) or some other patterned shirt and solid slacks. For the classic look, pick a shallow V-neck, one that allows the collar to pop, but stays folded in the sweater (so as not to look like you have the disco fever) hence the button collar.
I myself am going to pair it with a white John Varvatos slim fit button collar dress shirt and a nice pair of herringbone-ish Paul Smith wools slacks, but I will let you make your own choices, you are a big kid now.
Marc by Marc Jacobs has a great black cashmere shallow v-neck for just over $100 at their boutique on 4th and Bank in The Village