Joe,
High NOx results from excessive combustion chamber temperature. If you aren't failing by too much, there are a couple things you can do that might give enough improvement to pass. First, retard timing by pulling the octane jumper, and second get some high octane fuel. If you can get 94 that's good, or maybe even mix in a couple gallons of 100 octane unleaded race fuel. I think Sunoco even sells that at some service stations on the east coast.
BTW I don't know about MD, but here in CA you are labeled a "gross polluter" if you fail twice, so it's important to get a pre-test to make sure you will pass the second time!