How do I keep ice from melting too fast in a cocktail?

Use one large ice cube (2-inch square or larger) instead of multiple small cubes, freeze water in an insulated tray, and serve drinks immediately. Pre-chill the serving glass to slow initial melt.

Cubed ice melts proportional to its surface area. A 2-inch single cube has a much smaller surface area than ten 0.5-inch cubes of the same total volume, so the single large cube dilutes the drink much more slowly. Two practical techniques: (1) Large cube molds — silicone trays that make 2-inch cubes. Roughly $10 for a tray that produces 6 cubes per freeze. Use them for spirit-forward cocktails (Old Fashioned, Negroni) where slow dilution matters. (2) Pre-chill the glass. Put the rocks glass in the freezer for 10 minutes before mixing, or fill it with ice and cold water and pour it out right before assembling the drink. The cold glass means less heat to absorb from the liquid, and the ice melts dramatically more slowly. (3) Quality of ice. Tap water freezes with dissolved air, producing cloudy cubes that melt unevenly. Boiled-then-cooled water freezes cleaner. Spring water freezes cleanest. For an Old Fashioned or any cocktail where the ice is visible and central, ice quality matters more than people expect.