Get email subscriptions from Japanese travel agencies. They often run really good sales. I usually use IACE-USA and Amnet USA, but there are HIS and other places as well. They have Facebook, too, so worth checking. But keep in mind that if you want to wait for the sales, it will be about 3~4 months before your trip. So, if you are thinking about going in Spring, the sale will be sometime around January & February, which actually works out great for you since Japanese companies always throw a big sales during New Year time. You may want to check 2017 New Year sale price and that will give you a good estimate on how much it will cost in 2018.
From my last experience with IACE-USA:
I just scored round trip flight from my town(stopover in Chicago) to Tokyo to my birth place in Japan for $900.00/adult when IACE had a 1 day sale couple weeks ago. The flight is with UA but I was able to select ANA operated code share flight between ORD and NRT for this price. Considering we usually pay somewhere around $1300~1800/adult, it was a big success to me. I'm leaving in mid January but this sale was good till 3/31/17 departure... I believe. On the same 1 day sale, ads showed a roundtrip from NY to Tokyo starting from $558~.
Hope this helps!