Drive the Car
If you have a car that is older than 1999 than you will want to drive your vehicle for at least 20 minutes prior to arriving at the smog station. This will ensure that your vehicle is properly warmed up and is running at it's optimum level.
Tips to help you pass smog test
Make sure your tires are poperly and evenly inflated. Many states require a dynamometer test, where the technician positions the tires on rollers that allow the engine to run at high speeds while stationary. If your tires are underinflated the car's engine will have to work harder to reach the engine revolutions required by the test.
Change the Oil, only if needed
Dirty oil in the cranckcase could release additional polutants that can cause you to have a fail test reading. Make sure you check for any problems under the hood like to see if any hoses are cracked, broken or disconnected before heading to the shop.
Check Engine Light
Any check engine light is an automatic failure! You'll need to get a diagnosis and fix before you test. You're local mechanic can tell you what error code your check engine light is reading so you can fix the issue before you smog.
Check coolant and gas levels before the test.
Since the test runs the car at high speed while it is stationary, less air flows through the radiator to cool it. So make sure to fill your coolant tank properly. Also, the car will be on a dynamometer, possibly at an angle. If gas is low in the tank, this could expose the fuel pump and put vapor in the fuel line, causing the car to run out of gas.