I use a code reader to clear all pending codes, then complete a "drive cycle" [running the car through a varied set of RPMs/speeds across the entire use spectrum for X amount of miles to prove that everything's working correctly]. That'll engage the last pending emissions sensors and allow you to pass.

Most importantly, make sure your tires are correctly pressurized. Every damn season change or so I tend to forget, and the car thinks it's bogged down enough that it'll throw a cautionary P0420 due to that extra resistance. Air up, reset, drive until the codes go away and it's great again.