Ice is a very important component of most cocktails. When making any drink that has to be stirred or shaken with ice, it is important to have a lot of ice on hand.
If you do not have an ice machine at your disposal, then it is best to purchase bags of cubed ice from the grocery store for stirring and shaking as well as for serving cloudy drinks, highballs or any drinks at larger gatherings where an appropriate cache of large, clear ice will be harder to maintain.
For clear, spirit-forward drinks served on the rocks, such as Negronis and Old Fashioneds, large, clear cubes or spheres are preferable. This sort of ice can be purchased in bulk as artisanal ice or created using various home systems.
Some tiki drinks call for crushed ice. For these drinks, it is useful to own an ice bag and mallet so that you can crush up ice on your counter, but only if you are making a small number of drinks. Otherwise you will likely want an ice crushing machine of some kind.
If a drink is stirred or shaken with ice, that ice should almost never be the ice it is served with. The drink should be served over fresh ice, unless otherwise noted.