Guilin - Dazhai (Longji) - Yangshuo

It is true that there is special bus running from the south station to Dazhai. The bus departs at 08:00, 08:30, 09:30, 13:00, and 15:00. It costs CNY40 per person. But you may need to ask for your hotel to help you book it in advance. Well, there seems no direct bus from Dazhai to Yangshuo. You need go back to Guilin from Dazhai, and then take bus t Yangshuo.

Thank you so much, Jerry! But... how could I ask for a reservation? I won't be hosted in any Guilin (center) hostel, but in Yangshuo or Dazhai or the like. Do you see any possibility?

Maybe it's a crazy idea but... could I ask my Beijing hostel for it? :S

By the way I'll go to China in November... maybe I wouldn't need to book it in advance as it's low season (don't know).

As far as I know, there is a hotel in Dazhai can pick you up, but I forget its name. If you did not book that hotel and also do not want to stay in Guilin, it will be hard for you to take the special bus. You can choose to transfer.
It will be difficult to ask the hotels in Beijin to book that ticket for you.
It is not necessary to book in advance, but if you can, you had better book in advance, which will make the trip more smooth.