Don (and others)
My 90 ZR-1 passed our Texas (Dallas/Ft Worth) emissions test with no worries. It can be done. I'll offer these suggestions based on my experience with various cars over the years, ranging from turbo 4 cyls to American muscle to British 6's and V12's.
1. First and foremost, make sure you're running right. If something isn't right with the engine, more than likely it's not going to pass. This should seem obvious but I've got pals who will take something that spits and coughs and then wonder why it flunked. Running clean, tuned up, purring like the pussycat you know it can be - that's the way to take it to the sniffer.
2. If the engine was designed with cats, make sure you're running cats. Again, seems obvious, but...
3. If the car has been sitting for some time with old gas, get all that old gas out one way or another. Drain the tank or drive it until it's gone. Fill up with a nice tank of fresh clean gas. I'm very agnostic when it comes to brands. Some guys swear by a specific brand purporting to have a special additive. I've seen a tanker go from a Chevron to an off-brand convenience store, so I have little faith in the marketing. Just use something from a nice station. You know it when you see it. Drive the car through at least that tank.
4. See if you can find a bottle of RXP gas additive. It's a little bitty orange bottle that costs about $8. You'll swear it's too small to do anything. Trust me, it does something. It's a very concentrated formula that will knock the carbon off the innards. Add it to a fresh tank. Do not be alarmed if you blow a bunch of black smoke for the next tank of gas. Important: Do NOT take the car to the sniffer on that tank of RXP gas. Everything that it cleans out has to be expelled someplace... it's through the exhaust. Wait 'til your next tank.
5. Run the car around. Don't just take it up the block and back, drive it around. Drive it around town, to work, on errands, to Grandma's house, whatever. Get it good and warmed up and exercise it. Give that nice gas and RXP a chance to do some magic. When you're done with the treated tank, fill up with some nice "plain" gas (no additive).
6. Once you've taken care of all of the above and you're ready for testing day, make sure you arrive at the sniffer nice and warmed up. I like to drive around for at least an hour or so. Your engine will run far cleaner hot than it does just started up. I don't typically smell very sweet or behave at my best when somebody first wakes me, and my cars are sort of the same. We take a while to get all limbered up.
Now I'm not from California so I can't speak from personal experience there. And I know your standards are more stringent than what we've got here in the Lone Star State. But hopefully you've got everything running right under the hood. I think that if you are, and you follow some of the common sense rules here, you may have a good result at your emissions test.
Best of luck!
Note: here's more on RXP:
http://rxp.com/