I took for granted the social lifestyle my parents enjoyed while we kids were growing up, with people over every weekend and friends stopping in after work. But what would you expect from a house with a bar in the living room? Sometimes I felt like we should have a neon sign in the window. My parents were always ready for company. They stocked cold beer, snacks, and mixers. In the restaurant business, this is called mise en place ("put in place"), and I'm sure their propensity for it created the atmosphere to which company flocked.
Without getting all Martha about it, there are some easy ways for you to be prepared for impromptu summer cocktails.
Repeat after me: Good things come in all packages. Jars and cans will keep on the shelf for round after round of guests. Mini bottles of Looza fruit juices are my favorite mixers to stock. The apricot, pear, and mango juices make universal flavor donors to most spirits, including vodka, gin, or rum. R. W. Knudsen sells a plethora of booze-friendly mixers in the jar; the hibiscus cooler and pineapple-coconut require little work other than ice cubes and rum. Continuing with the lazy and prepared theme, Pacific Natural Foods, better known for gravies and soy products, sells raspberry and peach ice tea in Tetra Paks. Pour one into a pitcher with fresh mint and no one will know the difference.
Don't shun the freezer section. Although fresher may be better, the point is to be ready for guests. Cascadian Farm sells organic lemonade and raspberry juice concentrate in cylindrical cardboard cartons. You can get a high rate of return for little exertion by mashing some of the juice concentrate with frozen berries and liquor, then topping the drink off with soda. I also freeze half-gallons of Paul Newman's Own Limeade; by samurai-ing the top off and scraping some into a glass with rum or tequila, it doubles as an instant Slushee. In addition, any combination of these frozen ingredients can turn big bottles or boxes of mediocre white wine into awesome instant sangria.
Think beyond your regular soda mixers. Talking Rain has a new line of seltzers—spearmint mojito, pomegranate-lime, and citrus zest—aimed at the cocktail set. While I personally don't like drinks that taste like gum, the spearmint seltzer makes a cool instant mojito, without the watered-down mint taste these drinks usually have. Talking Rain's zest flavor has a true-to-type aroma that goes perfectly with a little juice and vodka.
Stock up on special sauces—things that can live in the fridge. A jar of Nellie & Joe's Key West Lime Juice or ginger juice from the Ginger People livens up any drink instantly. Having one high-quality flavored syrup, like Monin's Blackcurrant, on hand comes in handy for ades and iced teas (and can be used as a little rouge for cheap wine). I am a devotee of pomegranate molasses, which you can find at many specialty food stores, but R. W. Knudsen sells small jars of pomegranate concentrate that are easier to find.
The fantasy life of glossy magazines that manipulate you into wanting to install mint gardens for mojitos is fine and lovely. But in real life, you are statistically more likely to have people over if you're ready for them. Your guests will think you're such a clever host, and you won't have to lift a finger except to point to your mise en place and the ice. And that's a good thing.