After being stopped, be respectful to the officer and be aware of your constitutional rights.
Officers have a difficult job and attempt to be diligent in performing their duties (and arresting suspects). Some may give you a benefit of a doubt if you are respectful. They can also be rude, demeaning, and authoritative in the way they treat you. You must keep cool and not get into word-games, or be demeaning right back. With this mind, you have the right to remain silent, and not answer questions about drinking, driving (if the officer did not see driving), or anything else that could be used against you. You have the right to tell the officer you will cooperate but will not make a statement, and you have the right to not do any field sobriety tests. This may cause the officer to arrest you for suspicion of DUI (if you smell of alcohol and/or were driving recklessly) - however, you have not helped him or her to fill out his report by providing incriminating responses about your drinking pattern, quantity of alcohol, and other information the prosecutor will use against you.
If you are arrested, you must take one of the offered blood alcohol tests
In being licensed to drive in California, you have given implied consent to local law enforcement to test you for alcohol consumption. If you refuse to perform a blood, breath, or urine test at the station, then you are in violation of California implied consent laws. You face a one-year license suspension, and 48 hours in jail. To avoid these severe consequences, you MUST provide a sample to the police. Again, this does not mean you have to talk to the officers, incriminate yourself, or perform field sobriety tests on the street. Even if the sample you provide appears to be greater that .08% BAC, an experienced DUI attorney should be aware of various defense strategies involved in questioning the reliability of the results.
Finally, stay calm (this is easier said than done)! Keep in mind that everything you say or do will show up in a police report.