Chill glasses in advance in a fridge or simply fill with water and ice to chill while your are mixing your cocktail in the shaker. A chilled glass will keep your cocktail as cool as possible, from the first sip until the last.
That means, quality branded spirits, branded mixers, fresh fruit and freshly squeezed juices. If you put poor ingredients in you'll get a poor result out. Read more on stocking your bar...
Holding spirit bottles by the neck makes it much easier to pour exactly the right measure.
Add lots of fresh ice to the shaker immediately before shaking. Always add the ice last to avoid unnecessary dilution of the other ingredients.
By the time you finish a drink in a large glass it will have warmed up, especially if the drink is served "up" (without ice) in a martini glass.
Fresh fruits and juices can vary in sweetness so you may need to fine tune your cocktail with extra citrus juice or sugar syrup to achieve the right balance.
Use fresh garnish and simple cocktail picks. No garnish is far better than a forest of fruit, veg and umbrellas in the top of your drink!