Bar mix is a mixer made of sugar and lemon/lime juices. Pre-made bar mixes can be be bought from most places that sell spirits. It's also easy to make your own:
Recipes[]
Recipe #1[]
Take 1 cup of lemon juice and add 2 tablespoons of sugar. (You can also add a couple of egg whites per liter of mix if you want to make your cocktail foamy.)
Recipe #2[]
Half lemon juice, half lime cordial, splash of worcestershire sauce, sugar.