Contrary to popular belief, regular gas is preferable to get you over the top on an emissions test due to it burning easier than premium. Also, make sure the car is good and HOT before testing. This is kind of a moot point if your state does load testing but more important if you have to wait in line for a static test. Try picking a time when there's no line. Another thing I've done is to put a piece of cardboard infront of the radiator so that the temp gauge runs steady a little higher than normal.
Changing the plugs is a good idea because worn plugs can effect combustion. If the plugs aren't that old, make sure they're gapped properly. If they were run at all with older gas they can be fouled to some extent.
I went through this time and time again with my old white truck. It was always just over the limit. Finally, they did away with emissions testing for non-OBDII vehicles...
