Cocktail Info

Use 3 to 6 ice cubes
Hold the shaker with two hands in front of your shoulders and shake horizontally forth and back
Shake for about 10 to 20 seconds (up to 1 min for some drinks eg. a gin fizz)
Shake the shaker as hard as you can. Don't just rock it, you're trying to wake it up, not send it to sleep.
Always remember to shake the shaker and not yourself !!!
Never mix more than three drinks at a time

Ice is the cheapest "ingredient" when mixing drinks but it can have a great impact

Always use fresh, really ice-cold and dry ice
Don't waste too much time - it musn't start melting
Use the right amount of ice - if you use too much, you will get watery drinks
Don't store your ice for a long period in the refrigerator next to frozen food - it will absorb odors

There are basically two ways to produce crushed ice:

Electric or mechanical ice crushers: worth the price if you need a lot of crushed ice
A hammer and a towel: wrap some ice cubes in the towel and use the hammer to crush them

Use equal parts of sugar and water and heat this mixture in a saucepan till the sugar is dissolved. Let cool down.
Some people use up to two parts sugar and one part water (but this will result in sweeter drinks)

Whenever possible, use fresh fruit juices
ALWAYS use fresh lemon and lime juice (this might be the most important hint of all!)
Avoid bar mixes and artifical, concentrated substitutes like the plague

Garnishes can help making a drink look appealing, appetizing, tasty and refreshing
If something looks good, it will probably taste better but as with everything the right amount is the key: if it takes you minutes to get rid of the garnishes before you can start drinking, there is probably something wrong usually you would use as less garnishes as possible
There are drinks where garnishes are important ingredients: imagine a martini without an olive or a pimm's no.1 cup without a cucumber peel - you will certainly miss something and even taste the difference

Use at least medium quality brands and whenever possible use high quality brands.
This does not mean that you have to use champagne for $100 per bottle - if you want to serve a kir royal, with some experience you will find good products for reasonable prices.
If you think that you can't taste the difference because it is mixed anyway - try it and you'll see: a martini mixed with cheap gin and/or a cheap vermouth will taste disgusting!