Follow Steve Ringlee's suggestions on the basic tune up and vacuum leaks. Have you set the basic idle speed and mixture? The procedure is described in the FAQ. Pay particular attention to the timing. Sometimes retarding the timing a few degrees will bring NOx emissions into compliance.
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john

A lot depends on how bad the reading was. A small excess over the allowed can usually be addressed by the things in the previous post, and anything else that can reduce combustion temperatures, e.g., higher octane fuel. My daughter's Jetta recently failed NOX by a bunch. Did all the usual things, also replaced the oil with Mobil 1, and it brought the readings down, but still not far enough. What finally did the trick was a new cat.

In any emissions test failure, one has to eliminate the normal "tuneup" items first by making certain that the plugs/wires/cap and rotor/air filter/motor oil are in good shape.
Having done that, look for vacuum leaks in the intake system (cracked lines/leaking gasket/hoses left unconnected).