Glad you asked. There are many defenses, but it varies from country to country, and probably from state to state. DUI charges are very technical, therefore naturally the defenses have to be technical as well. I know it sounds shady to the average person, but if the charge itself is a technicality then the only way to get off is via a technicality. I can only speak for what I know about the Canadian system, but I would assume it's similar in the US. In Canada a DUI is typically two parts - Impaired Driving and Over 80. Impaired is when you're visibly impaired, and Over 80 is a technicality which just means you blew over .08, (80mg of alcohol in 100ml of blood).
A little background...here when you're pulled over by the police you have to blow into a roadside screening device, which will measure your breath and spit out a pass, caution or fail message. This measures your mouth alcohol. If you fail, (like I did), you're immediately arrested for suspicion of driving Over 80. Then you're transported to the station where after some time you do two official blows, twenty minutes apart, into their Intoxilyzer machine, which will give actual readings of your blood alcohol. If your result is higher than 80 you get charged.
Now the defenses...in Canada we have the Canadian Charter of Rights and Freedoms, (equivalent to the US Constitution), which has different sections and subsections, and if it's proven in court that your rights were violated by the police, then the breath test readings are excluded and defendant is acquitted. That's an example of Charter relief. I'm almost certain something similar would be available in the US. Of course these arguments are very slim, as most judges will allow a certain degree of violations to your rights. Really only the most egregious violations result in an acquittal. Ie. not being allowed to call a lawyer is a major one. Or not being tested at the station within two hours.
In my case, my lawyer tried a Charter defense which I botched with my testimony. He was trying to get the breath results excluded from evidence, arguing that the officer should not have tested me at the roadside until 15 minutes had passed, because I was coming from a night club that was very close to where I was pulled over, which was something I remembered telling the cop. According to the operations manual of the roadside screening device, the officer is supposed to wait 15 minutes to administer the test, if it's known or suspected that the person had just recently been drinking. He didn't wait, even though he knew I had just consumed alcohol from somewhere close by. The lawyer grilled the cop in cross examination, (I actually felt sorry for him), and had the manual included into evidence. This was supposed to set the stage for me. All I had to do was testify on the stand what I told the cop, that I had just come from a nearby club. On the stand, for some fucked up reason, I omitted the word "just". That killed the argument, and my lawyer was irate with me. After testifying we went into another room where he berated me and said I had just murdered an argument he spent two weeks trying to build. The omission of that one word meant that Charter argument was useless, because how was the cop supposed to know I had just come from somewhere close by, and therefore why should he have had to wait 15 minutes to test me? So that argument was dead in the water, but after calming down my lawyer told me not to beat myself up over it, as it was a 50/50 argument anyway. We still had my main defense.
My real trial proper defense was "bolus drinking" aka the "last drink" defense. This one is where you try to argue that your BAC was under the limit when you were actually driving. Because alcohol takes up to 30 minutes to be absorbed into the bloodstream, it opens the narrow defense that you were not Over 80 at the time of driving. For this to work you need to recall your drinking pattern of that evening, and establish that the majority of the drinks were consumed 15-30 minutes before you were driving. Then you need corroborating evidence. A witness helps, so does receipts if you were at a bar. I didn't have those things, even though I was with my friend who backed out of testifying, (because he was sick and died later. I didn't know he was sick at the time, I thought he'd just punked out). The most important piece is a toxicologist, who will go on the witness stand and go through your order of drinks to establish your BAC range when you were actually driving. My tox guy said my BAC was most likely under 50. He said my BAC was rising by the time I reached the station, something that was further corroborated through police notes, indicating my speech was slurred and eyes were red-rimmed at the time I blew into the Intoxilyzer. These symptoms were not present at the time of my arrest. I like to think my defense looked strong.
In a nutshell that's the strategy of a bolus drinking defense. The only remaining factor is to convince the judge. The only way for it to be believable, is if you are believable. The prosecutor's job is to say you're full of shit and attack your credibility, and if the judge takes his side, then you're fucked. I think that's what happened to me. The judge said my drinking pattern didn't make sense, and said that because I lied to the arresting officer, (I initially told him I didn't have anything to drink), it reflected negatively on my credibility. I find you guilty. So that's that.
A word or two about bolus drinking, in Canada this defense has been legislated out as of December of last year. So a person arrested for a DUI today wouldn't have that defense available anymore. Also legislated out last year was reasonable suspicion. So a cop doesn't even have to suspect you've been drinking in order to demand a roadside breath test. That's something I have to be cognizant of, seeing as how I have a record now. It's kind of scary that you can be pulled over at any time and for no reason. The best defense is to not drink and drive. Not sure if bolus drinking is still used in the US. It's a very difficult defense with a lot of moving parts, but if executed flawlessly it could result in an acquittal. Just didn't work out for me.
So that's what I know about the technical Over 80 charge. Impaired Driving proper is different, and according to my lawyer is much more difficult to defend, as it's less technical. A long time ago there existed the "I don't feel drunk" defense, which was legislated out ages ago.
Overall DUI charges in this country have an 80% conviction rate. Very difficult to defend, therefore very lucrative for defense lawyers who like to just throw shit on the wall, in the form of Charter defenses, and hope one sticks. They all talk a big game but are mostly full of hot air. My lawyer had over 40 years of experience, but that just led to a false sense of security. At the end of the day you have to create enough reasonable doubt to convince the judge to acquit you. You can probably tell I regret fighting it. Hindsight is 20/20, and I always wish I had just plead guilty in the first 90 days after getting arrested. I would've only faced a three month driving prohibition and nine months with the interlock. The whole shit would've been well into the rear-view by now, and my car insurance would've normalized. Every fiber of my being was telling me at the time to just plead guilty and get it over with, but I ended up listening to other people. I have a lot of affluent friends, and they all chimed in on what they would do if it happened to them, and they all shouted "fight it!" Of course none of them were paying the legal bill. So I took a chance and paid a hefty price. All the more reason to never do it again.