You can do a couple things to give yourself a better chance. I would call a few times a week to check to see if there are any openings for main seating on your cruise. From what I've been told, the waitlist is not immediately checked when something opens up, it's only checked once per day, so if something is open when you call, you'll get it as opposed to whomever is next on the waitlist. I'd call every day during the week just prior to the 60-day refund period, as this is when most cancellations will happen.
If this fails, there is a place and time set aside once you board where the dining manager will try to make changes for you. There's no guarantee here, though.
I've used the "call-every-few-days" method to change reservations a few times and it's worked every time.