Not being drunk at the time of a drunk driving stop is probably the best defense possible when facing drunk driving charges. If you can prove that you have not, in fact, consumed any alcohol prior to getting behind a wheel, you have a very strong defense against the charges. Even if you did consume alcohol, there are possible defense strategies that can defeat or help reduce charges and sentencing.
Some defenses, which aren't typically common, rely on the fact that a person was not driving out of a choice to do so. The driver might be doing so under duress; for example, if someone is holding a gun to a drunk person's head and making them drive, that would be a defense. If a person is driving drunk and does not realize they drank at all, they might have some type of defense. This defense might be valid is someone consumed a spiked drink or was otherwise tricked into drinking alcohol.
Other uncommon but possibly workable defenses include entrapment defense, or the fact that a law enforcement official requested someone drive while they were drunk, and necessity, which occurs when someone is driving drunk because any other option is a greater evil.
More commonly, drunk driving defenses attack the procedures by which a charge originated. Defenses might look at whether officers made a proper stop, used field and breath tests appropriately and kept testing equipment properly maintained. Sometimes a person might claim that their blood alcohol levels were appropriate while they were driving, but had risen by the time a test was administered. Understanding the best defense strategy for your case takes experience and knowledge of the law.
Source: FindLaw, "Defenses to Drunk Driving," accessed Sep. 04, 2015