To mix great cocktails, you must have knowledge of the ingredients you are working with. Understand the effect and hour of a cocktail. Always be aware of hour of the day. Certain drinks excel at certain times. For example, a Bloody Mary is more appropriate at 8AM than a Jack and Coke.
Temperature is important.
Drinks should be served in stemmed glasses and handled from the base to avoid transfering heat to the cocktail. Timing is important. Someone once said that you want your cocktail while it is still laughing at you. If you are making drinks for several people, make the drinks in batches rather than individually. Never refill a glass.