Always Have Your Documents Readily Available
You know the cops are going to ask for it, so always keep your License, Registration and Insurance Card where you can get them quickly and without fumbling. Fumbling is one of the 'facts' the cops are going to use to support probable cause to arrest you. The point of these tips is to help you fight the DUI/DWI charges, not to avoid going to jail. If you've been drinking, going to jail is probably inevitable. But you should act to preserve your rights and act to aid your defense wherever you can.
Don't Answer Their Questions
Never make any statements that may admit guilt. When the cops ask you how much you've been drinking, what/when you last ate, where you're coming from or how fast you were driving, answer that "I've been advised by a lawyer not to answer questions like that," or by answering the police with a non-smart ass question. An exception to this would be if you are under medication or have a medical condition, like GERD or diabetes that may make defending against a DUI/DWI easier. Let the cops know if one of these apply.
Don't Take the Field Sobriety Tests
There is no rule saying you have to submit to the field sobriety tests. They can only hurt you. The pass/fail of the test is entirely subject to the officer's (who's predisposed to arrest you) opinion about whether you pass. WARNING: If you do not take the FSTs you will go to jail, but the cops will have much less evidence against you (particularly with respect to the driving while impaired charge).
Don't Agree to Take the Preliminary Alcohol Screener (Breathalyzer)
This is the handheld breathalyzer the cops carry at the scene. They are entirely unreliable. And will only be used as proof you were drunk and impaired. You do not need to take the PAS if you are over 21. If you take the test, it will be used against you and most probably will never be able to help you avoid arrest. If you are under 21, you must take the PAS test when required by a cop.
Take the Blood Test, Not the Breath Test
There is much more that can go wrong with the blood test than the breath test, increasing your chances to beat the charges at the trial/DMV.
Use One of Your Phone Calls to Call a Friend
When given the opportunity to make a phone call from the police station call a friend. Talk to him. Allow him to hear that you are not slurring your words or having trouble talking. Also, if given more than one phone call, call your own phone leave a message that can be played back at trial/DMV hearing to show you weren't incoherent or slurring your words.
Use Your Passengers as Witnesses
Have them turn around in the car and observe you out the back window. They can be witnesses to rebut the officer's statements about your behavior at the scene.
Take Note of the Circumstances of Your Arrest
I know that being arrested is scary. But it will help if you pay attention to the surroundings. After you are released and go home, write down everything that happened while it's fresh in your mind.