Other Asian, Australian and South Pacific Frequent Flyer Programs - CA 77W on the SHA-PEK route?

I fly this route about 2-3 times per month and have been trying to try their 77W product. Every time I thought I booked the 77W on the CA site, I end up on a 332 or other aircraft.

I know air carriers move around their aircraft due to demand but how do I improve my chances?

Thanks much.

Try to travel at peak times for this route when they can sell more seats?