You need a good channel knife. Though an essential garnishing tool, the difference in use between the varying styles may not become clear to you until you give them a really good test drive.
Bibulo.us currently endorses the funny looking, but great cutting Zyliss Zester as the channel knife of choice for the home. Lots of control, gentle on the hand, and performs like a champ even for exceptional twist-generating projects like New Year's Eve Champagne Cocktails for the whole party.
If you don't like the way the Zyliss cozies up smoothly into your palm, do get yourself a more traditional style channel knife to help you garnish appropriately. One major reason to use this tool is so you can cut a twist safely right over the glass and achieve the proper spray of citrus oils across the top of the drink.
The excellent channel knife showdown by Kevin Ludwig, of Beaker and Flask, in the July/August 2007 issue of Imbibe Magazine (the existence of such an article was the clincher for our getting a subscription) gives more tips on fail-safe twists as well as the pros & cons of four other knives. Kevin's pick was the Zyliss, for which advice we are most grateful. Back issues of Imbibe are cheap and this one also has a great cover story by Paul Clarke of The Cocktail Chronicles on obscure vintage ingredients and a short piece by Camper English, noted Alcademic, on throwing spirit tasting parties.