Most boozy slushies are made by chucking some booze, mixers, and ice into a blender. It's a simple process, but not a foolproof one, and can result in an overly icy, crunchy beverage. To prevent shards of frozen water from jacking up your vibe, make your slushies in an ice cream machine.
Photo by Ken Hawkins.
I've tried this myself, and I love this method for a lot of reasons. Not only does it allow you to omit ice entirely, preventing the dilution of your beverage and making for smoother concoction, it provides another use for your ice cream maker, further justifying its noble existence. Food 52 has an excellent recipe for making piña coladas this way, but it can be applied to almost any cocktail (like a Negroni!), just be sure to chill everything ahead of time for easy freezing.