There is however a temptation for overkill - fruit, plastic umbrellas, swizzle sticks etc... Such a concoction would simply look a mess.

No, by adding just the correct amount of garnish, the wise bartender achieves the perfect balance, whilst giving a visual indication of what is to be expected in terms of flavour. 
For example, a slice of lemon or lime indicates a sharp, crisp cocktail is awaiting, whilst a maraschino cherry or ball of melon indicates a sweet cocktail.

Clearly there are some cocktails that have a traditional garnish added to them (the olive in a dry martini). In some cases (such as a margarita) the glass rim is frosted with salt. This is best done by first wetting the rim.

Fruit

Fruit plays a vital role in cocktail making. Most cocktails require some fruit in them, and it is a useful thing to keep a fair amount of fresh fruit available for parties and other occasions.

Fruit is important for adding eye appeal, but can also (in moderation) be an exciting base for cocktails. Hopefully some of these photographs will provide inspiration for you.