The first and most important rule, when you want to decorate a cocktail is:
Don't overdue the decoration! This means that you don't want to get the cocktail undrinkable by putting so much decoration on the glass, so that the guest can't drink the cocktail in a normal way. Most cocktails that are popular in the whole world don't have more than a slice of lemon on them.

Refreshing long drinks and highballs have more decoration when the glass gets bigger. A cocktail stick is standard with these drinks. Ladies get a straws in ther drink. In this way they don't ruin their makeup when they are drinking.

some examples of decorations.

You can buy a whole assortment of plastic animals, umbrellas, and so on for the decoration of a cocktail. In addition to this, slices of fruit, strips of orange or lemon peel, mint twigs, etc. can also be used.

CoolCocktail » handbook » decorations