Measure out each ingredient called for by your recipe. Your MEASURE choice is important. Shot glasses, double sided hammers, and other generic measures don't do the job precisely enough. Their markings are too often on the outside. You can't see through metal, and glass is refractive and therefore inaccurate.

POUR each measured LIQUOR into an EMPTY SHAKER - NO ICE!! The liquor running over the ice will melt the ice and dilute the drink before it ever reaches the glass. This is the single biggest mistake bartenders make. So next time you see your local bartkeep pack a tumbler with ice and start to free pour liquor, raise your hand and politely say "Do me a favor. . . it will help your tip." 

After all liquors and juices and accents such as bitters are in the shaker, add ICE. Add enough that your ingredients are covered twice over. Do Not pack the body of the shaker with ice - there needs to be room for the ice and liquids to move separately. It is the passing by each other that creates the desired chill. In real time shakes, this will be between 15-20 for most drinks served without ice. For drinks that will be served with ice, 4-6 shakes is enough to get the ingredients mixed and a basic chill set.

SHAKE, adverb: the way in which you move your forearm such that the shaker in your hand moves with enough tempo to occassion the ice and liquids within to move past each other arhythmically a pace not unike that of a tango.
Or for those of you from Arkansas, don't shake the thing like a chicken that you are trying to kill in the fewest rapid jerks.
We are trying to chill and integrate the ingredients without destroying their integrity or creating a maelstrom that would sink the Pequod.
Cook shakins, mahn

You will begin to feel the chill on your shaker. This is why glass or metal is important. You want to feel the drink directly. Some drinks, think whiskey for example, you will want a bit chilled but not much, others you will want icey. In fact depending upon the degree of chill and number of ingredients, you may come to stir your drink in a rocks glass, add a cube or two and just swirl occassionally as you drink.
Remember, the colder your drink, the less flavor from the ingredients and the sharper it drinks. Water and alcohol freeze at different temperatures AND cold depresses the sweet sensation.
Once it is chilled - and most Cocktails need to be thoroughly chilled despite my aside above - remove the cap from the pour spout and pour into a proper and prepared GLASS. Shake gently at the end as ice will block the last run of your drink sometimes. I will sometimes have to pour on the run, but try your best to pour directly into a glass standing on a counter.

Frankly a well made drink does not need a garnish. We all know how irritating it is to remove parsley and other inedible snippets from our dinner plates. But if you can add something that accents the drink, fine.

And by accent I mean complement the flavor profile of the drink. That does NOT mean add a wedge of watermelon to a watermelon martini [if you ever make such a drink you lose all visitation privileges to this site]. If the drink is sweet, add a clensing garnish - if the drink is bitey, add a sweet garnish. The goal is to keep every sip of the drink balanced and tasting exactly as the previous one. A drink that changes over the course of drinking is a poorly made drink.

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