Have you decided to go to China already? If yes, then book a ticket now as the earlier the cheaper! , You know it is the hot season to Harbin in Winter. You'd better book with a Chinese travel agency as they offer the biggest discount. This is a good travel agency i think, you can book through them. January is the winter holiday, there will be a lot of people in China then, you should book the hotel and tickets i advance. Actually, the best way is to follow a travel agency, they will arrange everything for you before you come to China, what to need to do is enjoying the beautiful scenery of China. Just my personal opinion.
PS: Travelchinaguide is really a good and reliable travel agency.

As the data in the next few months, the ticket is relatively cheaper in late August and early October (Beijing to Harbin). Just hard to tell exct time and where to buy. It also depends on airlines. Seems China Southern Airlines are cheaper. You can pay attention to flight infor changing through internet.