Vehicle emmisions is a scam. I got a 1990 Toyota Camry that gets over 30 MPG. It barely passes emmisions every 2 years. Had give up my 2nd car a few years ago because it could not pass. It was a 1984 Lebaron 4 cylinder that got about 24 MPG. The gov. wants me to pay 25,000 for a new Obamamobile.
You can buy gas additives at any auto-parts store that will help you pass.
They are typically added to the gas tank a week before the expected test, the car driven as per the instructions (and not otherwise), and the whole tank consumed before the test. Then, other additives are added to the next tank, and you're ready to test.
A car mechanic can help you with this scrubbing process; he can de-carbonize the engine as a first step.
I think it helps if you use an oxygenated fuel (gasohol) or use an alcohol-based additive (methanol, isopropanol -- if you're desperate, you can even try 91% rubbing alcohol [isopropyl alcohol = isopropanol] -- when you go to test. Again, an experienced mechanic's advice will help.
By the way, don't go dumping isopropyl alcohol, etc., in your tank on a regular basis. It has side effects that are not good: it tends to dissolve the plastic anti-corrosion lining sprayed on the inside surface of your gas tank, for instance. Once in a while is okay, but don't make it a habit.