If you're willing to take a chance on Delta after their hellish early August computer tangles, this may be a good time. AirfareWatchdog's Cheap Flights page is offering round-trips NYC-SEA for $278 for mid-September, with some dates as low as $261.
Similar fares are showing for October, with Delta as the lowest. Looking ahead on the same site, November is American's month on the route at $275; December through February the low fares are at around $300, with Alaska, JetBlue and American taking turns as lowest.
Other availables, all RT pricing from NY, with dates variable: Chicago $76, Fort Lauderdale $85, Dallas $93, Myrtle Beach $132 and Detroit $140.