Measuring is essential to consistently balanced cocktails (and therefore delicious cocktails). The tool most professional bartenders use for this task is called a jigger, you can read more about the different types and sizes available on the Jiggers page. These are the most convenient option for home bartenders as well, but you can measure with just about anything that holds liquid. Measuring cups and teaspoons work just fine. The most important thing is that you measure.
There isn’t much of a technique to measuring. You just fill and pour. Though when using jiggers, particularly the two-sided cone-shaped style, there are a few good habits to adopt and pitfalls to avoid which will ensure you’re as accurate, clean and quick as possible (which is especially important if you’re a professional bartender).
The method demonstrated below is with these double-sided jiggers, but the rules generally apply regardless of the style of jigger you use.
Fill it to the top. Ideally there should be a little bubble of meniscus on the rim. Don’t be afraid if a few drops spill off. They will from time to time. That’s what paper towels are for.
Quickly tip and pour! On to the next ingredient.
Fill that Jigger Up All the Way!
Be sure to fill the jigger all the way up to the rim or line. Particularly with cone shaped jiggers, which have larger volume discrepancies towards the top. The difference of a couple millimeters above or below a line can add up to as much as quarter ounce, which will have a major impact on the cocktail. A graduated cylinder proves this point very effectively.
Hold the jigger upright. It should be alongside the mixing vessel. Be careful to avoid the two bad habits outlined below. Begin to fill.
Don’t hold the jigger over the mixing vessel. While it’s tempting because it will keep spills off the counter, you’ll run into trouble when you have to tip and pour and probably end up spilling more.
Don’t hold the jigger at an angle. Many do this to anticipate pouring. But you’ll never be able to fill the jigger all the way up. So it’s less accurate and you could throw off the balance of the drink.
Measuring in Barspoons & Teaspoons
Some cocktail recipes call for a barspoon's worth of an ingredient. Which can be confusing because not all barspoons are the same size. In general, I'd say:
1 barspoon equals about ¾ teaspoon, or a heavy ½ teaspoon.
I try to be specific with my recipes, and list many smaller quantities in teaspoons. But there's still something romantic to me about measuring in barspoons, so I don't abandon it as a unit completely.
Of course this would all be different if we were using the metric system. But alas, I live in the United States, and this is all I know.