A good bartender knows that making a great cocktail is more than just mixing the right ingredients. The perfect cocktail has to have the best quality ingredients, in the right proportions, served at the right temperature, in the right glassware with the right garnish. One of the most underappreciated and overlooked aspects of mixing drinks is the ice. It is one of those things we just take for granted, even though it is a crucial ingredient in most drinks.
Considering it is just frozen water, ice is deceptively complicated. If your ice is too big, it will not cool the drink fast enough, if it is too small it will melt too quickly and water down the drink. If you have too little ice, you will add too much mixer to fill the glass, too much ice and your customers think you are trying to rip them off.
Should you use crushed ice or cubed ice? In the best of worlds you have both available to you. Crushed ice blends easier in blended drinks and cools shaker drinks much faster, but cubed ice will not melt as fast and distributes more evenly in a glass. You also need to think about where you are storing your ice. Unless you have an ice maker or dispenser right at your bar, if your ice melts too quickly you will constantly be running to the freezer or ice maker to refill your ice. Rounded ice cubes (which are not really cubes) are better than square ones because they are less likely to stick together which can cause problems for both the bartender and the drinker.
One final point, make sure you have a good strong metal ice scoop. Ice will inevitably stick together and a metal scoop can be used to break up your ice, while plastic ones will often break. Never scoop up ice directly with a glass. If the glass breaks, you will have to dump all your ice out or else risk serving somebody a shard of broken glass.