• If it is a close friend, try and use a soft, calm approach at first. Suggest to them that they've had too much to drink and it would be better if someone else drove or if they took a cab.
• Be calm. Joke about it. Make light of it.
• Try to make it sound like you are doing them a favor.
• If it is somebody you don't know well, speak to their friends and have them make an attempt to persuade them to hand over the keys. Usually they will listen.
• If it's a good friend, spouse, or significant other, tell them that if they insist on driving, you are not going with them. Suggest that you will call someone else for a ride, take a cab, or walk.
• Locate their keys while they are preoccupied and take them away. Most likely, they will think they’ve lost them and will be forced to find another mode of transportation.
• If possible, avoid embarrassing the person or being confrontational, particularly when dealing with men. This makes them appear vulnerable to alcohol and its effects.

--Source: Colorado Department of Transportation