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 cube has about a quarter of the surface area of the same volume in half-inch cubes — 24 square inches against 96 — so the single large cube dilutes the drink far 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, so the ice melts more slowly. (3) Clarity of ice. Cloudy ice is caused by air and minerals trapped as the cube freezes inward from every side at once. Clear ice is denser, has no air pockets, and melts more slowly. The fix is directional freezing: freeze water in a small insulated cooler with the lid off, so it freezes from the top down and pushes the air downward, then cut the clear upper section into cubes. Boiling the water first makes only a marginal difference on its own, and bottled or spring water makes none — clarity comes from how you freeze the water, not what water you use.