Ice is one of the most important ingredients in a cocktail. Even when it's not in the glass it's used during mixing. This makes the quality of ice very important.
First of all the water used has to be of good quality. If you don't have good quality tap water I would strongly recommend that you filter the it. This greatly improves it. This is important since badly tasting water will affect the taste of the drink.
Secondly the temperature of the ice is important. I always take my ice directly from the freezer when putting it into the shaker or the glass. I never use ice that has been out on the bar for a while since it gets wet. Wet ice dilutes the drink to much.
Third is the size. If I don't use crushed ice, which I don't very often, I used large chunks. I freeze ice in freezing boxes of various sizes. Most are five by ten by two centimetres. But some are about ten by ten by ten centimetres. I then use a cast iron pester to crack the ice into suitable chunks. A small block is usually made into two pieces. One piece like this is enough when shaking a cocktail.
The reason the size is so important is that it makes it possible to control dilution more precisely. Since the ice is so large it will cool the drink faster whilst melting slower. This makes it possible to mix the cocktail until it's perfectly diluted without worrying about its temperature. The same is true when serving on the rocks. The larger the chunks of ice the slower they melt and the better they keep the cold.
But when serving on ice it also provides for a nice visual effect. Imagine your sour or old-fashioned poured into a glass with one single block of ice that is so large it just fits in the glass. Or being served a long drink on hand cracked pieces of ice, not one looking like the other. It always impresses my guests 🙂