When making mixed drinks, first add ice, then liquor and then your mixers. The average pour of alcohol is 1.5 ounces (or a "jigger"), but depending on your mixers and audience, it can go anywhere from 1 to 2.
A new touch in cocktail bars is the addition of a "condiment tray" of small bottles filled with lemon juice, lime juice, simple syrup and bitters — so your guests can adjust their drinks as they see fit.
As a general rule, stir clear (or translucent) drinks; shake opaque ones.
If you're serving a drink on the rocks, larger cubes of ice are often appreciated, as they melt more slowly than shaved ice, diluting the drink less.
Get to know the spirits you're using; not all gins (or vodkas or whiskeys) are created equal, and some might do better with certain blends than others. For example, when making a sweeter cocktail like the Old-Fashioned or the Manhattan, you'll want a stronger whiskey to give the drink backbone.