I agree with advance booking, but it may not help you avoid the crowded scenes at the BJX ticket halls.

Now that there are machines that issue tickets to Chinese with ID cards, they've closed the special windows for ticket pick-up in many Chinese stations. You just have to wait in the regular ticket lines if you don't have the ID card. I don't know specifically the situation at BJX, but my working assumption would be that the special window no longer exists and that you'll have to wait a bit in line to pick-up the ticket you ordered.