I've been flying between China and the US almost exclusively on miles (in biz) for the past 1.5 years, and have gotten pretty good at finding seats. The trick is focus on flights that: 1) have large business class cabins; and/or 2) routinely have unused inventory.

JFK-HKG is probably the best example in both cases (half the plane is J, and there are 5 flights per day). Direct flights between the US and Haneda are a close second.

On *A, I've had good luck with IAD-NRT on NH, and UA to KIX is often on offer as a last resort (when redeeming miles, I now try my best to avoid UA and AA on long flights at all costs).