Tonic gives a hint of bitterness to either gin or vodka.
Seltzer water adds body to flavored vodkas for a light, low-key sipper and sparkle to a heavy drink.
Cola is a classic, and goes equally well with whiskey as it does with rum. (If you want to be fancy, squeeze a wedge of lime into your rum and cola and call it a Cuba Libre.)
Ginger ale and ginger beer go well with bourbon and blended whiskey. If you like it sweet, use ginger ale--for more spice, use ginger beer.
Lemon-lime soda . though a little sweet, is a good beginner's introduction to whiskey drinks and works well in fruity punches.
Grapefruit soda mixes perfectly with tequila and a pinch of salt.
Orange juice works with gin, vodka and rum — freshly-squeezed adds another layer of texture and flavor.
Grapefruit juice is better with gin than vodka, as the florals of gin and the sharpness of the juice complement each other nicely and is always a great match with rum and tequila.
Lime juice is the prime ingredient in sour mix; bottled sweetened lime juice is perfect with gin.
Cranberry juice works well with vodka and orange liqueur — it also gives the cosmopolitan its signature red color.