Making a great garnish is a lot like making any recipe: the best results come from the best ingredients.  And remember, while a great garnish does not necessarily guarantee an amazing cocktail, it will add to the visual appeal!
When making cocktail garnishes, always use fresh, washed ingredients.  Fruit is not only a decoration, but also affects the taste of the cocktail if presented properly.
Citrus fruits like lemon, lime and orange give more juice if you first soak them in warm water for a few minutes.  Then roll them on the cutting board against your hand.  This allows the fruits to become softer before squeezing juice out of them.
Cut fruits should be covered with a damp napkin or stored in a covered container when not in use.  Generally cut fruits will not last longer than a day or so.

TYPES OF GARNISHES

RIM
Margaritas and daiquiris are served in glasses that are rimmed with chocolate powder, herbs and spices (salt, pepper and cinnamon) or shredded coconut.  Prepare a small plate with the desired  ingredient.  With a fruit wedge, dampen the rim of the serving glass (use an orange wedge for sweet rimming and a lemon wedge for others).  Gently dip the reim of the glass in the plate.  Add the cocktail mixture to the glass.  If using shredded coconut or chocolate powder, dampen the rim of the glass with honey, icing or sugar syrup before following the earlier procedure.
WEDGE
Many tropical drinks are served with a wedge of fruit, usually lemon, balanced on the rim of the glass.  Wedges not only look nice, they’re functional too: by squeezing the wedges, guests can add extra lemon to their drink without getting they hands covered in juice or pulp.  Citrus fruit, apples, peaches, strawberries, or any other fruit that will not break easily are usually preferred as wedges.  Pineapple slices make great wedges for tropical drinks.  Rip off the green part of the pineapple, cut off the ends, and then slice the pineapple into 1 cm thick rings.  Cut these rings into 4 to 6 slices with slits for placing in the glass.  Aloha! (some fruits will brown quickly.  To delay this process, soak them in ice water with a dash of lemon or lime juice, or squeeze citrus juice onto the fruit.)
BALL
Balled fruit makes a great tasting addition to a cocktail or punch.  Use fruits like watermelons, which generally do not brown or spoil quickly and can be frozen.  Use a melon baler to scoop small spheres of fruit.  Prepare in advance and add to drink just before serving.
DROP
This very simple decoration is often used to identify a cocktail.  For example, a raisin would be used with an apple martini and a coffee been in a coffee martini.  Vegetables, candy, nuts, jelly beans, dried fruit, cherries, berries, a piece of chocolate or anything similar can be used.  Drop the garnish in a glass and cover with a clear or translucent cocktail; the garnish should sink to the bottom.
SPIRAL
Citrus fruit spirals can greatly enhance the visual appeal of a cocktail.  Cut fruit in half length-wise, and slice long strips no wider than ¼ inch or ¼ cm.  Soak in ice water.  Wrap strips tightly around a straw and freeze.  Alternatively dip strips one at a time in boiling water and then return immediately to ice water; they should curl up naturally.  Use as required.
GRATED OR SHREDDED
Grated coconut, chocolate, ginger, nutmeg or cinnamon is ideal to sprinkle as a garnish over thick drinks or those topped with whipped cream or a scoop of ice cream.
JEWEL
This is very simple and elegant decoration is commonly used in martinis and cosmopolitans.  Use a toothpick or a skewer stick and put it through olives, or small vegetables (onions or tomatoes), cherries or berries, and drop it into the glass.
 
sourced from cooking & more march-april 2007
by Delnaaz Irani