Not sure what you mean by "best way", but I've had the best luck upgrading to C from JFK as opposed to DFW and ORD. I know that's completely devoid of any statistical relevance, but you could easily check ExpertFlyer to look at C availability for the dates you're interested in. I'd recommend booking and calling sooner rather than later, as C availability can reappear (if zeroed out already) before your date of travel. Also, don't just keep an eye on it and try to upgrade at the last minute, as your already hefty copay will be increased depending on how close to date of travel you are.