Ice is essential for all types of mixed drinks, and it should always be clean and clear.
Usually you would add ice to a glass before pouring any ingredients into it. This cools the liquids and prevents splashes.
There are also different ways to use ice.
Most mixed drinks call for ice cubes to be used, which can be bought by a local supplier if it suits your needs. You can also make your own, however creating ice cubes that are as crystal-clear as those from a supplier can often be difficult. Some tips:
Use softened water filtered by a reverse osmosis system.
Use warm water, which contains less dissolved gas.
Reduce the temperature of your freezer so that the ice forms slowly, reducing the problem of dissolved air bubbles.
You may be able to purchase this at your local store. However, a cheap alternative is to wrap the ice in a clean towel, lay it across a suitable surface and break it into small pieces with a mallet or appropriate instrument.
Apply the same technique as above, however the ice should be broken more finely. You can also buy ice machines that will do the job for you. Crushed Ice is usually used in tall drinks, drinks that require straws, frappes etc.