Mixers
Buying mixers is often the most complicated part of building a home bar. Do you really need ginger ale? And what are bitters anyway? Here are some basics that you should always keep on hand, but check your favorite recipes to ensure you have all the proper ingredients:

Vermouth
Dry and Sweet. Essential for classic Martinis and Manhattans.

Liqueurs
So many liqueurs, so little time. These include Schnapps and cordials. You'll need to make some choices from the dizzying array of liqueurs available. If Fuzzy Navels are a favorite, Peach Schnapps are a must. How about a White Russian? You'll need Crème de Cacao for that! Narrow down the list to those that you'll use most often, and then add the rest over time.

Soda
Tonic and soda water, cola (regular and diet), lemon-lime (a.k.a. 7-Up or Sprite)

Juice
Lemon, lime, orange, grapefruit, (spicy) tomato, cranberry, and pineapple

Margarita, Piña Colada, Daiquiri Mix
Frozen cans are convenient and can be stored for months