At its simplest, a Highball comprises only four elements - glass, ice, spirit and carbonate but temperature and dilution are what makes or breaks a Highball.
Use a frozen or at least a chilled tall, narrow (in proportion to its height) glass with straight or slightly flaring sides. Highball glasses have a capacity to their brim of 6oz/177ml to 10oz/295ml with 8oz/235ml to 9.5oz/280ml being optimal.
Many think a tall slim 12oz/355ml glass, the kind that will just accommodate a 330ml bottle/can of beer or carbonate, is a Highball glass. Such glasses (over 10oz/295ml) in capacity are actually Collins glasses and NOT Highball glasses.
As David Embury says in his The Fine Art of Mixing Drinks, "Don't expect to have the Highball which you order at a bar served in anything larger than a Sour glass [3-5oz] unless you order a double. It is a decidedly generous bar that gives you a full 2-ounce drink in a Highball (the measuring glass may look like 2 ounces, but note the way the bottom of the glass is constructed). Two ounces of liquor in a 14 or 16 ounce glass filled with ice and carbonated beverage would more nearly resemble the traditional Sunday-school lemonade than a Highball."
Ideally use (double frozen) ice drawn directly from a freezer rather than wet surface ice form a bar's ice-well or ice machine. To quote Charles H. Baker, "A lukewarm drink means a lukewarm guest."
Use frozen or at least a chilled bottle of spirit. After all, many a good bar has gin and vodka in the freezer to help ensure they serve really cold Martinis so why not also scotch whisky for Highballs?
In Esquire, Dave Wondrich writes, "Finally, pour in the sparkling water (club soda or seltzer). If at all possible, this should be refrigerated in order to keep the ice from melting prematurely and drowning the bubbles. How much fizz? Less than twice the amount of hooch is too strong, more than three times too weak."
For me, equal parts, or perhaps 2 parts whisky to 2½ parts soda makes for the perfect Scotch Highball. During this drink's heyday, many demanded only a small amount of soda water to be added, a "Scotch and a splash" was a common bar call.
Some add a further unnecessary complication by stirring a Highball. This is unnecessary as the relatively low density of the spirit poured first versus the heavier carbonate poured on top produces a self-mixing combination. As Esquire's 1949 Handbook for Hosts explains, "Spare the spoon and save the drink. The slightest contact with silverware squelches the bubbles - which do the mixing job unassisted. If one of your guests is stir-crazy, give him a plastic or glass swizzle stick."
There is nothing to say that a Highball can't comprise more than just ice, spirit and a carbonate, indeed at Joerg Meyer's Boilerman bars such simple Highballs are in the minority with infusions, liqueurs and aromatized wines taking the Highball to new levels of sophistication. As Joerg Meyer says, "We take a cocktail and Highball it."