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.
Thank you very much for the info. I always try to keep my car in good running shape. The Camry would not start on Easter Sunday night,missed work. Too dark to work on. Got up at 7 AM Monday, lifted plastic cover on pos. terminal. Found corrisive build up. Cleaned it out and car started right up.