Shake when there's citrus, dairy, egg, or any non-spirit component that needs aeration. Hard shake for 10-12 seconds. The shaker tin should sound like the ice is breaking; if it's silent, you're shaking too gently. Two-tin Boston is house standard. Hold the small tin at the bottom, the big tin on top, point the small tin slightly away from the guest. Crack the seal by striking the heel of your palm on the bigger tin where it meets the small one. Never shake a stirred drink (Manhattan, Negroni, Martini). Never stir a sour.