1 - The DWI investigation begins the moment your vehicle is first spotted by the officer, and
continues on to the moment you are placed in a cell.
Officers look for indicia of intoxication from the moment they first spot your vehicle. Things like swerving while you reach for something you dropped on your floorboard or driving slow while you double check your GPS directions can be incriminating.
After the stop, something as simple as fumbling for your license or insurance card can be used against you as well. Even after you're arrested the officers are still gathering evidence and you'll most likely be on camera, so don't yell or curse or otherwise act like you're out of control.
2 - The officer must have probable cause for the stop.
An officer cannot pull you over unless you have violated a traffic law or he has some other reason to suspect that you have been involved in criminal activity. An exception to this is a valid check point stop. In order for a check point stop to be valid, the check point must meet several constitutional requirements.
3 - You don't have to answer any questions beyond identifying yourself.
We're all taught to be polite, and it seems rude to not answer a direct question that is asked of you, but you should know that "anything you say can and will be used against you." The officer himself will tell you this when he's placing you under arrest, but it's true from the very first moment you are pulled over.
You have the right to remain silent. Exercise that right by refusing to answer any questions that may be incriminating.
4 - There's a difference between the breath test they give you on the side of the road, and the one they give you back at the station.
Most officers carry a Portable Breath Test device (PBT) that they will ask you to blow into on the side of the road before placing you under arrest. This is not the true breath test that you impliedly consent to by Missouri Law. The PBTs are not presumed to be dependable and are only used by the officers to help build probable cause, not as actual evidence of your blood alcohol level.
Refusing to submit to a PBT will not result in the loss of your license. The best way to know the difference between the PBT and the real breath test is that usually the PBT is requested before you are placed under arrest, and the officer will read you your Implied Consent Warning before the real test, letting you know that refusing will result in a one year revocation of your driving privileges.
5 - You don't have to submit to the Field Sobriety Tests.
Most likely, if an officer has asked you to submit to Field Sobriety Tests, he has already made up his mind to arrest you. Submitting to the tests only gives him more evidence to use against you. You can politely refuse to submit to these tests.