Layering is key for the presentation of many drinks, and hobbyists will argue that certain drinks must be layered to properly "set off" the taste of each layer as you sip the cocktail.What they may not tell you is that it is quite difficult to do!
Layering is density dependent. Any solution has a specific density. Those with higher density sink through those with lighter densities, and the lighter density solutions will tend to remain floating above the high density liquids. Thus the key to knowing how the layers will turn out is by consulting a density chart.
The problem with alcohols is that many of them have densities that are very similar. This means that layers won't form if there is too much mixing or turbulence, and therefore, layering has to be done very slowly and very carefully.
The easiest way to layer a drink is by using a medium plastic syringe with a reasonably sized bore. Simple suck up the required amount and slowly add it to the glass by placing the tip of the syringe on the inside of the glass. Slowly is the key, and a syringe gives you excellent control. However, this is best done discreetly or with specialty syringes that don't look medical syringes. After all, it isn't too professional looking, and could possibly embroil you in a bad situation if everyone isn't aware of what is in the syringe.
Therefore, the common technique today is to pour the liquor slowly (always slowly!) over the back of a spoon, touching the side of the glass. This (hopefully) allows the liquor to trickle into the drink without too much downward disturbance, and settle nicely upon the top of the drink.
Here is a step-by-step for layering using the spoon method: 1. Consult the density chart for relative densities of your ingredients. 2. Add your most dense ingredient first. 3. Place a teaspoon upside down into the glass, up against the edge of the glass and close to the surface of the drink. 4. Slowly pour the next heaviest liqueur over the back of the spoon. Raise the spoon slowly if necessary. 5. Continue with each ingredient, ending with the least dense.