Don't just "take turns," if no one is pregnant, an AA veteran, or volunteers. In an ideal world, everyone would be equally capable of abstaining from alcohol in the midst of a wild shindig. However, you know that some of your pals are heavier partiers than others, and it would be foolish (as well as unfair) to name them the designated driver. On New Year's Eve, especially, this task should fall to the most trustworthy person, no matter if he was the designated driver at the Christmas party two weeks ago.
Show some respect. Being the designated driver isn't all that much fun to begin with, and it's even less so when your friends say things like "Hey, Eric, this is a really great Mai Tai! Too bad you can't have one!" or "What's that you're drinking, buddy? Ginger ale? Party on, dude!" Remember, your designated driver is there to (potentially) save your life-not to be an object of ridicule.
Keep a watchful eye. If your designated driver isn't much of a drinker to begin with, you can pretty much trust her to act responsibly. But if you had no choice but to assign the job to someone who'd just as soon be rip-roaring drunk, check in occasionally to see that she isn't cheating. It's easy for a designated driver to believe one drink early in the evening won't matter, except that "early evening" has a way of creeping into "late evening" and from there to well past midnight.
If you're the host-ask. Hosting a New Year's Eve party involves more than just laying out chips and guacamole and keeping the fridge well-stocked with beer. It's also your responsibility to insure your guests make it home safely. When a group of partiers arrive, it's appropriate to ask (in a breezy way, of course) which of them is the designated driver. If no one has given this any thought, tell your pals that you won't be serving them any drinks until they sort things out.
Be prepared to call a cab. Every now and then, the best-designed designated driver plans go awry. If your designated driver is in no shape to drive you home, don't appoint the person who's "least drunk" to climb into the driver's seat. Call a car service (there are plenty of these active on New Year's Eve), and return the next day to pick up your vehicle.