Prevention is the first step. As you probably already know, the first area of the brain to be affected by alcohol is the area that controls judgment. Once a person starts to drink, reason and judgment go out the window so trying to reason with a potential drunk driver is incredibly difficult. Alcohol also gives a person a false sense of ability, giving them the mistaken belief that they aren't as drunk as you may believe and they are capable of driving safely. To keep from getting to this point, friends who plan an event that includes drinking should agree beforehand on how they will get home safely. If a decision is made before you go out drinking, chances are that you will stick with that decision. You should also remember that it can take a very long time for someone to sober up. Hot coffee will not work. Alcohol has to be metabolized by the liver and that takes time.
Designating one or two people as designated drivers who agree not to drink at all is the best option for all. Many restaurants and bars strongly promote designated driver programs and will provide free soft drinks and, in some cases, appetizers and other premiums to the designated driver to keep them sober and allow them to get their friends home.
Taking some sort of public transportation such as a bus, train, or a cab is also an effective choice where they are available. Brewing companies such as Miller/Coors and Heineken have partnered with cab companies throughout the US to create 1-800-TAXICAB. This is an easy to remember phone number that will put you in contact with a local cab company. It's not free but it's a whole lot cheaper than a DUI conviction. Some localities have programs like the Support Anna program in the Puget Sound area of Washington State that has partnered with bars to give free cab vouchers to get drinkers home safely. Check your area to see if such a program exists. You can also check with the restaurant/bar you plan to patronize to see if they have a similar program.
If you find yourself in a position of trying to prevent someone from driving after they have been drinking, the National Highway and Traffic Safety Administration offers the following tips:
If all else fails, call the police. The police can be much more persuasive and they would rather prevent a drunk from getting behind the wheel than to respond to the scene of an alcohol related crash.
If the situation becomes heated and feelings are hurt, remember that, once reasoning returns after the person sobers up, it will probably all be quickly forgotten. Remember also that a temporary loss of friendship is preferable to the permanent loss of a friend through a drunk driving crash.
You've heard it before but it bears repeating; "Friends don’t let friends drive drunk."